powerpc/boot: Only use '-mabi=elfv2' with CONFIG_PPC64_BOOT_WRAPPER

When CONFIG_PPC64_ELF_ABI_V2 is enabled with clang through
CONFIG_PPC64_BIG_ENDIAN_ELF_ABI_V2, building the powerpc boot wrapper
in 32-bit mode (i.e. with CONFIG_PPC64_BOOT_WRAPPER=n) fails with:

    error: unknown target ABI 'elfv2'

The ABI cannot be changed with '-m32'; GCC silently accepts it but clang
errors out. Only provide '-mabi=elfv2' when CONFIG_PPC64_BOOT_WRAPPER is
enabled, which is the only way '-mabi=elfv2' will be useful.
Tested-by: "Erhard F." <erhard_f@mailbox.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230118-ppc64-elfv2-llvm-v1-1-b9e2ec9da11d@kernel.org

arch/powerpc/boot/Makefile

@@ -40,6 +40,9 @@ BOOTCFLAGS    := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
 
 ifdef CONFIG_PPC64_BOOT_WRAPPER
 BOOTCFLAGS	+= -m64
+ifdef CONFIG_PPC64_ELF_ABI_V2
+BOOTCFLAGS	+= $(call cc-option,-mabi=elfv2)
+endif
 else
 BOOTCFLAGS	+= -m32
 endif
@@ -61,9 +64,6 @@ BOOTCFLAGS	+= -mbig-endian
 else
 BOOTCFLAGS	+= -mlittle-endian
 endif
-ifdef CONFIG_PPC64_ELF_ABI_V2
-BOOTCFLAGS	+= $(call cc-option,-mabi=elfv2)
-endif
 
 BOOTAFLAGS	:= -D__ASSEMBLY__ $(BOOTCFLAGS) -nostdinc