Add compilation target android-arm64 (#208)

* Add android-arm64 support

cd bgfx
export ANDROID_NDK_ROOT=$NDK_ROOT
export ANDROID_NDK_ARM=$NDK_ROOT/toolchains/aarch64-linux-android-4.9/prebuilt/linux-x86_64
export ANDROID_NDK_CLANG=$NDK_ROOT/toolchains/llvm/prebuilt/linux-x86_64
make android-arm64

* Expect ANDROID_NDK_ARM64 to define the compilation aarch64 toolchain

Using _ARM64 will generate less conflict in configuration files than
reusing the _ARM var.

For example, to setup an android-arm64:

export ANDROID_NDK_ROOT=$NDK_ROOT
export ANDROID_NDK_ARM64=$NDK_ROOT/toolchains/aarch64-linux-android-4.9/prebuilt/linux-x86_64
export ANDROID_NDK_CLANG=$NDK_ROOT/toolchains/llvm/prebuilt/linux-x86_64

make android-arm64

makefile

@@ -8,6 +8,7 @@ GENIE=../bx/tools/bin/$(OS)/genie
 all:
 	$(GENIE) vs2017
 	$(GENIE) --gcc=android-arm gmake
+	$(GENIE) --gcc=android-arm64 gmake
 	$(GENIE) --gcc=android-mips gmake
 	$(GENIE) --gcc=android-x86 gmake
 	$(GENIE) --gcc=mingw-gcc gmake
@@ -27,6 +28,14 @@ android-arm-release: .build/projects/gmake-android-arm
 	make -R -C .build/projects/gmake-android-arm config=release
 android-arm: android-arm-debug android-arm-release
 
+.build/projects/gmake-android-arm64:
+	$(GENIE) --gcc=android-arm64 gmake
+android-arm64-debug: .build/projects/gmake-android-arm64
+	make -R -C .build/projects/gmake-android-arm64 config=debug
+android-arm64-release: .build/projects/gmake-android-arm64
+	make -R -C .build/projects/gmake-android-arm64 config=release
+android-arm64: android-arm64-debug android-arm64-release
+
 .build/projects/gmake-android-mips:
 	$(GENIE) --gcc=android-mips gmake
 android-mips-debug: .build/projects/gmake-android-mips

scripts/toolchain.lua

@@ -50,6 +50,7 @@ function toolchain(_buildDir, _libDir)
 		description = "Choose GCC flavor",
 		allowed = {
 			{ "android-arm",     "Android - ARM"              },
+			{ "android-arm64",   "Android - ARM64"            },
 			{ "android-x86",     "Android - x86"              },
 			{ "asmjs",           "Emscripten/asm.js"          },
 			{ "freebsd",         "FreeBSD"                    },
@@ -221,6 +222,21 @@ function toolchain(_buildDir, _libDir)
 			premake.gcc.llvm = true
 			location (path.join(_buildDir, "projects", _ACTION .. "-android-arm"))
 
+		elseif "android-arm64" == _OPTIONS["gcc"] then
+
+			if not os.getenv("ANDROID_NDK_ARM64")
+			or not os.getenv("ANDROID_NDK_CLANG")
+			or not os.getenv("ANDROID_NDK_ROOT") then
+				print("Set ANDROID_NDK_CLANG, ANDROID_NDK_ARM64, and ANDROID_NDK_ROOT environment variables.")
+			end
+
+			premake.gcc.cc   = "$(ANDROID_NDK_CLANG)/bin/clang"
+			premake.gcc.cxx  = "$(ANDROID_NDK_CLANG)/bin/clang++"
+			premake.gcc.ar   = "$(ANDROID_NDK_ARM64)/bin/aarch64-linux-android-ar"
+
+			premake.gcc.llvm = true
+			location (path.join(_buildDir, "projects", _ACTION .. "-android-arm64"))
+
 		elseif "android-x86" == _OPTIONS["gcc"] then
 
 			if not os.getenv("ANDROID_NDK_X86")
@@ -893,6 +909,33 @@ function toolchain(_buildDir, _libDir)
 			"-Wl,--fix-cortex-a8",
 		}
 
+	configuration { "android-arm64" }
+		targetdir (path.join(_buildDir, "android-arm64/bin"))
+		objdir (path.join(_buildDir, "android-arm64/obj"))
+		libdirs {
+			"$(ANDROID_NDK_ROOT)/sources/cxx-stl/llvm-libc++/libs/arm64-v8a",
+		}
+		includedirs {
+			"$(ANDROID_NDK_ROOT)/sysroot/usr/include/aarch64-linux-android",
+		}
+		buildoptions {
+			"-gcc-toolchain $(ANDROID_NDK_ARM64)",
+			"--sysroot=" .. path.join("$(ANDROID_NDK_ROOT)/platforms", androidPlatform, "arch-arm64"),
+			"-target aarch64-none-linux-androideabi",
+			"-march=armv8-a",
+			"-Wunused-value",
+			"-Wundef",
+		}
+		linkoptions {
+			"-gcc-toolchain $(ANDROID_NDK_ARM64)",
+			"--sysroot=" .. path.join("$(ANDROID_NDK_ROOT)/platforms", androidPlatform, "arch-arm64"),
+			path.join("$(ANDROID_NDK_ROOT)/platforms", androidPlatform, "arch-arm64/usr/lib/crtbegin_so.o"),
+			path.join("$(ANDROID_NDK_ROOT)/platforms", androidPlatform, "arch-arm64/usr/lib/crtend_so.o"),
+			"-target aarch64-none-linux-androideabi",
+			"-march=armv8-a",
+			"-Wl,--fix-cortex-a8",
+		}
+
 	configuration { "android-x86" }
 		targetdir (path.join(_buildDir, "android-x86/bin"))
 		objdir (path.join(_buildDir, "android-x86/obj"))
@@ -1202,6 +1245,12 @@ function strip()
 			"$(SILENT) $(ANDROID_NDK_ARM)/bin/arm-linux-androideabi-strip -s \"$(TARGET)\""
 		}
 
+	configuration { "android-arm64", "Release" }
+		postbuildcommands {
+			"$(SILENT) echo Stripping symbols.",
+			"$(SILENT) $(ANDROID_NDK_ARM64)/bin/aarch64-linux-android-strip -s \"$(TARGET)\""
+		}
+
 	configuration { "android-x86", "Release" }
 		postbuildcommands {
 			"$(SILENT) echo Stripping symbols.",