powerpc: Allow CONFIG_PPC64_BIG_ENDIAN_ELF_ABI_V2 with ld.lld 15+

Commit 5017b459 ("powerpc/64: Option to build big-endian with ELFv2
ABI") restricted the ELFv2 ABI configuration such that it can only be
selected when linking with ld.bfd, due to lack of testing with LLVM.

ld.lld can link ELFv2 kernels without any issues; in fact, it is the
only ABI that ld.lld supports, as ELFv1 is not supported in ld.lld.

As this has not seen a ton of real world testing yet, be conservative
and only allow this option to be selected with the latest stable release
of LLVM (15.x) and newer.

While in the area, remove 'default n', as it is unnecessary to specify
it explicitly since all boolean/tristate configuration symbols default
to n.

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-v1-3-b9e2ec9da11d@kernel.org