Add a SCons option for Mold linker

[LinuxUserGD]

Describe the project you are working on

With LDFLAGS="-fuse-ld=mold" and LD="ld.mold" env vars it's possible to compile any project with gcc/clang compiler while using mold as linker: https://github.com/rui314/mold.git

Describe the problem or limitation you are having in your project

Currently there are only llvm=yes, lld=yes and thinlto=yes scons options to define compiler/linker.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Add scons option mold=yes for linking final executable with mold.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Change a/platform/x11/detect.py and add scons option / env args for mold linker.

If this enhancement will not be used often, can it be worked around with a few lines of script?

Adding LINKFLAGS="-fuse-ld=mold" to scons command line should work too.

Is there a reason why this should be core and not an add-on in the asset library?

Proposal is about build system configuration when compiling godot from source.

[Calinou]

Currently there are only llvm=yes, lld=yes and thinlto=yes scons options to define compiler/linker.

These are actually use_llvm=yes, use_lld=yes and use_thinlto=yes.

That said, now might be a good time to change the options to enums to avoid incompatible choices:

• Replace use_lto=yes and use_thinlto=yes with lto=none|thin|full.
• Replace use_lld=yes with linker=default|ld|gold|lld|mold.