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 d2b30cd ("ARM:
8384/1: VDSO: force use of BFD linker").

Commit fe00e50 ("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 055efab ("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.

Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: I2f6b29acdb7b80120599db61d1ffbaa0b5850ed2
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: ClangBuiltLinux/linux#1033
Link: https://lore.kernel.org/r/20201020011406.1818918-1-natechancellor@gmail.com

arch/arm64/kernel/vdso32/Makefile

@@ -14,16 +14,21 @@ CC_COMPAT_CLANG_FLAGS := --target=$(notdir $(CROSS_COMPILE_COMPAT:%-=%))
 
 CC_COMPAT ?= $(CC)
 CC_COMPAT += $(CC_COMPAT_CLANG_FLAGS)
+
+ifneq ($(LLVM),)
+LD_COMPAT ?= $(LD)
+else
+LD_COMPAT ?= $(CROSS_COMPILE_COMPAT)ld
+endif
 else
 CC_COMPAT ?= $(CROSS_COMPILE_COMPAT)gcc
+LD_COMPAT ?= $(CROSS_COMPILE_COMPAT)ld
 endif
 
 cc32-option = $(call try-run,\
         $(CC_COMPAT) $(1) -c -x c /dev/null -o "$$TMP",$(1),$(2))
 cc32-disable-warning = $(call try-run,\
 	$(CC_COMPAT) -W$(strip $(1)) -c -x c /dev/null -o "$$TMP",-Wno-$(strip $(1)))
-cc32-ldoption = $(call try-run,\
-        $(CC_COMPAT) $(1) -nostdlib -x c /dev/null -o "$$TMP",$(1),$(2))
 cc32-as-instr = $(call try-run,\
 	printf "%b\n" "$(1)" | $(CC_COMPAT) $(VDSO_AFLAGS) -c -x assembler -o "$$TMP" -,$(2),$(3))
 
@@ -107,15 +112,10 @@ dmbinstr := $(call cc32-as-instr,dmb ishld,-DCONFIG_AS_DMB_ISHLD=1)
 VDSO_CFLAGS += $(dmbinstr)
 VDSO_AFLAGS += $(dmbinstr)
 
-VDSO_LDFLAGS := $(VDSO_CPPFLAGS)
 # From arm vDSO Makefile
-VDSO_LDFLAGS += -Wl,-Bsymbolic -Wl,--no-undefined -Wl,-soname=linux-vdso.so.1
-VDSO_LDFLAGS += -Wl,-z,max-page-size=4096 -Wl,-z,common-page-size=4096
-VDSO_LDFLAGS += -nostdlib -shared -mfloat-abi=soft
-VDSO_LDFLAGS += -Wl,--hash-style=sysv
-VDSO_LDFLAGS += -Wl,--build-id
-VDSO_LDFLAGS += $(call cc32-ldoption,-fuse-ld=bfd)
-
+VDSO_LDFLAGS += -Bsymbolic --no-undefined -soname=linux-vdso.so.1
+VDSO_LDFLAGS += -z max-page-size=4096 -z common-page-size=4096
+VDSO_LDFLAGS += -nostdlib -shared --hash-style=sysv --build-id=sha1
 
 # Borrow vdsomunge.c from the arm vDSO
 # We have to use a relative path because scripts/Makefile.host prefixes
@@ -175,8 +175,8 @@ quiet_cmd_vdsold_and_vdso_check = LD32    $@
       cmd_vdsold_and_vdso_check = $(cmd_vdsold); $(cmd_vdso_check)
 
 quiet_cmd_vdsold = LD32    $@
-      cmd_vdsold = $(CC_COMPAT) -Wp,-MD,$(depfile) $(VDSO_LDFLAGS) \
-                   -Wl,-T $(filter %.lds,$^) $(filter %.o,$^) -o $@
+      cmd_vdsold = $(LD_COMPAT) $(VDSO_LDFLAGS) \
+                   -T $(filter %.lds,$^) $(filter %.o,$^) -o $@
 quiet_cmd_vdsocc = CC32    $@
       cmd_vdsocc = $(CC_COMPAT) -Wp,-MD,$(depfile) $(VDSO_CFLAGS) -c -o $@ $<
 quiet_cmd_vdsocc_gettimeofday = CC32    $@