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

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.

.github/workflows/linux_builds.yml

@@ -31,12 +31,12 @@ jobs:
             proj-conv: true
             artifact: true
 
-          - name: Editor with doubles and GCC sanitizers (target=debug, tools=yes, float=64, tests=yes, use_asan=yes, use_ubsan=yes, ld.gold)
+          - name: Editor with doubles and GCC sanitizers (target=debug, tools=yes, float=64, tests=yes, use_asan=yes, use_ubsan=yes, linker=gold)
             cache-name: linux-editor-double-sanitizers
             target: debug
             tools: true
             tests: true
-            sconsflags: float=64 use_asan=yes use_ubsan=yes LINKFLAGS="-fuse-ld=gold"
+            sconsflags: float=64 use_asan=yes use_ubsan=yes linker=gold
             proj-test: true
             # Can be turned off for PRs that intentionally break compat with godot-cpp,
             # until both the upstream PR and the matching godot-cpp changes are merged.
@@ -46,12 +46,12 @@ jobs:
             # Skip 2GiB artifact speeding up action.
             artifact: false
 
-          - name: Editor with clang sanitizers (target=debug, tools=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, use_lld=yes)
+          - name: Editor with clang sanitizers (target=debug, tools=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)
             cache-name: linux-editor-llvm-sanitizers
             target: debug
             tools: true
             tests: true
-            sconsflags: use_asan=yes use_ubsan=yes use_llvm=yes use_lld=yes
+            sconsflags: use_asan=yes use_ubsan=yes use_llvm=yes linker=lld
             bin: "./bin/godot.linuxbsd.tools.64.llvm.san"
             build-mono: false
             # Skip 2GiB artifact speeding up action.