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