powerpc: Fix use of '-mabi=elfv2' with clang

'-mabi=elfv2' is not added to clang's invocations when
CONFIG_PPC64_ELF_ABI_V2 is enabled, resulting in the generation of elfv1
code, as evidenced by the orphan section warnings/errors:

  ld.lld: error: vmlinux.a(arch/powerpc/kernel/prom_init.o):(.opd) is being placed in '.opd'
  ld.lld: error: vmlinux.a(init/main.o):(.opd) is being placed in '.opd'
  ld.lld: error: vmlinux.a(init/version.o):(.opd) is being placed in '.opd'

To resolve this, add '-mabi=elfv2' to CFLAGS with clang. This uncovers
an issue in the 32-bit vDSO:

  error: unknown target ABI 'elfv2'

The ELFv2 ABI cannot be used when building code for a 32-bit target. To
resolve this, just remove the '-mabi' flags from the assembler flags, as
it was only needed for preprocessing (the _CALL_ELF macro) but this was
cleaned up in commit 5b89492c ("powerpc: Finalise cleanup around ABI
use").

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-v1-2-b9e2ec9da11d@kernel.org