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

arch/powerpc/Kconfig

@@ -616,8 +616,7 @@ config PPC64_BIG_ENDIAN_ELF_ABI_V2
 	bool "Build big-endian kernel using ELF ABI V2 (EXPERIMENTAL)"
 	depends on PPC64 && CPU_BIG_ENDIAN
 	depends on CC_HAS_ELFV2
-	depends on LD_IS_BFD && LD_VERSION >= 22400
-	default n
+	depends on LD_VERSION >= 22400 || LLD_VERSION >= 150000
 	help
 	  This builds the kernel image using the "Power Architecture 64-Bit ELF
 	  V2 ABI Specification", which has a reduced stack overhead and faster