Linux: Build with use_static_cpp=yes by default for x86_64

This enables `-static-libgcc -static-libstdc++` which help make custom Linux
builds more portable (official builds have been using this option for years).

For some obscure reason Ubuntu 18.04 i386 crashes when using the option for
i386 builds, so let's play it safe and enable for x86_64 only for now.

platform/linuxbsd/detect.py

@@ -64,7 +64,7 @@ def get_opts():
         BoolVariable("use_llvm", "Use the LLVM compiler", False),
         BoolVariable("use_lld", "Use the LLD linker", False),
         BoolVariable("use_thinlto", "Use ThinLTO", False),
-        BoolVariable("use_static_cpp", "Link libgcc and libstdc++ statically for better portability", False),
+        BoolVariable("use_static_cpp", "Link libgcc and libstdc++ statically for better portability", True),
         BoolVariable("use_coverage", "Test Godot coverage", False),
         BoolVariable("use_ubsan", "Use LLVM/GCC compiler undefined behavior sanitizer (UBSAN)", False),
         BoolVariable("use_asan", "Use LLVM/GCC compiler address sanitizer (ASAN))", False),
@@ -390,4 +390,7 @@ def configure(env):
 
     # Link those statically for portability
     if env["use_static_cpp"]:
-        env.Append(LINKFLAGS=["-static-libgcc", "-static-libstdc++"])
+        # Workaround for GH-31743, Ubuntu 18.04 i386 crashes when it's used.
+        # That doesn't make any sense but it's likely a Ubuntu bug?
+        if is64 or env["bits"] == "64":
+            env.Append(LINKFLAGS=["-static-libgcc", "-static-libstdc++"])