SCons: Refactor Linux linker options with linker=<bfd|gold|lld|mold>

[akien-mga]

The new option is linker and lets the user specify the argument to
the-fuse_ld= linker flag directly. The supported options are:

• default: No change, typically uses GNU ld (bfd) unless the user or
  distro picked a different default /usr/bin/ld.
• bfd: GNU ld from binutils
• gold: GNU gold from binutils
• lld: lld from LLVM
• mold: mold, an extremely fast modern linker, not (yet) intended for
  use in production but great for development speed. Provided by distro
  mold package or needs to be compiled from source and installed to
  /usr otherwise.

Removes the use_lld=yes option, and make lld actually usable with GCC
too.

Not all the above are compatible or recommend for LTO, we recommend
using GNU ld with GCC LTO, or lld with LLVM ThinLTO.

• Implements and closes Add a SCons option for Mold linker godot-proposals#4531.
• Supersedes Added Scons Option for the Mold Linker #61433.

[akien-mga]

As a side note, @Calinou suggested this in godotengine/godot-proposals#4531 (comment):

Replace use_lto=yes and use_thinlto=yes with lto=none|thin|full.

I think that makes sense, could be done in a follow-up PR.

[Riteo]

Is this really needed? I always use LINKFLAGS="-fuse-ld=mold" and it works fine. I see this as a compiler dependent feature tbh

[akien-mga]

Is this really needed? I always use LINKFLAGS="-fuse-ld=mold" and it works fine. I see this as a compiler dependent feature tbh

Well the same can be said of most other build options. You can set CC and CXX instead of use_llvm=yes, etc., and LINKFLAGS="-fuse-ld=lld" instead of use_lld=yes which we currently have. Pass CCFLAGS="-flto" LINKFLAGS="-flto" instead of use_lto=yes, etc.

We just abstract them in a way that makes them either to use. Personally I always have to double check the exact syntax of the link flag so IMO having an option we can validate makes a lot of sense.

It's also more discoverable (we can document it, and you can see it in scons --help). I'd wager a significant proportion of people who compile Godot don't know they can use a faster linker than the default GNU ld.

[Riteo]

Looks fine!