Revert stack "[Driver] Add support for GCC installation detection in Baremetal toolchain (llvm#121829)"

This reverts the following stack of commits, due to them breaking the
Fuchsia toolchain and corresponding LLVM buildbot.

Revert "[Driver] Fix Arm/AArch64 Link Argument tests (llvm#144582)"
This reverts commit a79186c.

Revert "[Driver] Add option to force undefined symbols during linking in BareMetal toolchain object. (llvm#132807)"
This reverts commit 9cb7545.

Revert "[Driver] Fix link order of BareMetal toolchain object (llvm#132806)"
This reverts commit 31523de.

Revert "[Driver] Add support for crtbegin.o, crtend.o and libgloss lib to BareMetal toolchain object (llvm#121830)"
This reverts commit ec230aa.

Revert "[Driver] Add support for GCC installation detection in Baremetal toolchain (llvm#121829)"
This reverts commit eb31c42.

Author: mysterymath

clang/docs/Toolchain.rst

@@ -347,8 +347,3 @@ workarounds for issues discovered in libstdc++, and these are removed
 as fixed libstdc++ becomes sufficiently old.
 
 You can instruct Clang to use libstdc++ with the ``-stdlib=libstdc++`` flag.
-
-GCC Installation
-=================
-Users can point to their GCC installation by using the ``-gcc-toolchain`` or by
-using ``-gcc-install-dir`` flag.

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -847,9 +847,6 @@ def note_drv_available_multilibs : Note<
   "available multilibs are:%0">;
 def err_drv_multilib_custom_error : Error<
   "multilib configuration error: %0">;
-def warn_drv_multilib_not_available_for_target: Warning<
-  "no multilib structure encoded for Arm, Aarch64 and PPC targets">,
-  InGroup<DiagGroup<"multilib-not-found">>;
 
 def err_drv_experimental_crel : Error<
   "-Wa,--allow-experimental-crel must be specified to use -Wa,--crel. "