Fix linker invocation on OpenBSD.

Firstly, we need to explicitly request pthreads and also need to
mind the fact ports (like SDL2) install in /usr/local/lib.

Secondly, since OpenBSD 7.4 the system enforces indirect branch
targets on hardware platforms that support it. Until the LLVM
integration in Odin can be changed to emit proper branch targets
(e.g. endbr64 on amd64), we need to request the linker make the
resulting program opt-out of enforcement.

src/linker.cpp

@@ -701,6 +701,13 @@ gb_internal i32 linker_stage(LinkerData *gen) {
 					// This points the linker to where the entry point is
 					link_settings = gb_string_appendc(link_settings, "-e _main ");
 				}
+			} else if (build_context.metrics.os == TargetOs_openbsd) {
+				// OpenBSD ports install shared libraries in /usr/local/lib. Also, we must explicitly link libpthread.
+				platform_lib_str = gb_string_appendc(platform_lib_str, "-lpthread -Wl,-L/usr/local/lib ");
+				// Until the LLVM back-end can be adapted to emit endbr64 instructions on amd64, we
+				// need to pass -z nobtcfi in order to allow the resulting program to run under
+				// OpenBSD 7.4 and newer. Once support is added at compile time, this can be dropped.
+				platform_lib_str = gb_string_appendc(platform_lib_str, "-Wl,-z,nobtcfi ");
 			}
 
 			if (!build_context.no_rpath) {