FROMLIST: arm64: vdso32: Allow ld.lld to properly link the VDSO

As it stands now, the vdso32 Makefile hardcodes the linker to ld.bfd
using -fuse-ld=bfd with $(CC). This was taken from the arm vDSO
Makefile, as the comment notes, done in commit d2b30cd4 ("ARM:
8384/1: VDSO: force use of BFD linker").

Commit fe00e50b2db8 ("ARM: 8858/1: vdso: use $(LD) instead of $(CC) to
link VDSO") changed that Makefile to use $(LD) directly instead of
through $(CC), which matches how the rest of the kernel operates. Since
then, LD=ld.lld means that the arm vDSO will be linked with ld.lld,
which has shown no problems so far.

Allow ld.lld to link this vDSO as we do the regular arm vDSO. To do
this, we need to do a few things:

* Add a LD_COMPAT variable, which defaults to $(CROSS_COMPILE_COMPAT)ld
  with gcc and $(LD) if LLVM is 1, which will be ld.lld, or
  $(CROSS_COMPILE_COMPAT)ld if not, which matches the logic of the main
  Makefile. It is overrideable for further customization and avoiding
  breakage.

* Eliminate cc32-ldoption, which matches commit 055efab3120b ("kbuild:
  drop support for cc-ldoption").

With those, we can use $(LD_COMPAT) in cmd_ldvdso and change the flags
from compiler linker flags to linker flags directly. We eliminate
-mfloat-abi=soft because it is not handled by the linker.

Link: https://github.com/ClangBuiltLinux/linux/issues/1033


Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>

Bug: 141693040
Test: BUILD_CONFIG=common/build.config.gki.aarch64 ./build/build.sh
Link: https://lore.kernel.org/linux-arm-kernel/20201013033947.2257501-1-natechancellor@gmail.com/


Change-Id: I8a7e5736294a2cb9b60edcfdddb4937003fe6c01
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>