lang/ocaml: fix build on powerpc64 elfv2

LLVM miscompiles ocaml on powerpc64 elfv2:
ld: error: bytecomp/printlambda.ml:23:(.text+0x16): relocation R_PPC64_TOC16_DS out of range: 73024 is not in [-32768, 32767]

Use GCC instead on powerpc64.

Add a conditional patch enabled only on powerpc64 elfv2 to use elfv2 abi instead of elfv1.

Add a bunch of other patches to correctly check for abi instead of relying on endianness.

PR: 242389
Approved by: michipili@gmail.com (maintainer)