Added macOS x86_64 build target (osx-x64).

[labolado]

Title

Added macOS x86_64 build target (osx-x64).

Body

Problem

The bgfx makefile only provides osx-arm64 gmake targets. There is no gmake target for building x86_64 binaries on macOS, which is needed for creating universal binaries (lipo-merged fat binaries) that support both Intel and Apple Silicon Macs.

Root cause

The makefile's projgen and build sections were only populated with ARM64 macOS targets.

Fix

Add gmake-osx-x64 project generation and debug/release build targets, mirroring the existing osx-arm64 structure.

• projgen: add $(GENIE) --gcc=osx-x64 gmake
• Build targets: osx-x64-debug, osx-x64-release, osx-x64

Test

• Run make projgen on macOS and verify .build/projects/gmake-osx-x64 is generated.
• Run make osx-x64 and verify both debug and release configurations compile successfully on an Intel Mac (or via Rosetta).
• Use lipo -create to merge osx-arm64 and osx-x64 outputs into a universal binary.

[mcourteaux]

I think the reason this is not the case in the project as is, is because advanced users should manually invoke GENie either way. Relying on the makefile with a random set of project parameters is not a good idea either way.

If you're looking to build fully cross platform, I recommend the zig compiler toolchain. It's quite a bit of work to get it to compile bgfx and all it's dependencies, but once you're there, it's actually great. Pro tip in that case: don't compile shaderc through zig, just make that a native build and invoke it from zig.