[clang] [llvm] [PowerPC] Support -fpatchable-function-entry (PR #92997)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> So PPC64 can use ELFv2 for `Triple::OpenBSD`. We probably want to diagnose this OS for PPC64, since with ELFv2 we might emit separate local and global entry points which means only certain values can be passed to `-fpatchable-function-entry` option.

This patch matches the intended end result: NOPs before the global entry and after the local entry.

The current way GCC handles `N,M` where M>0 is undesired and will hopefully be soon changed: https://gcc.gnu.org/pipermail/gcc-patches/2024-May/651025.html , matching what I suggested in 2022: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99888#c5

https://github.com/llvm/llvm-project/pull/92997