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

[Chen Zheng via cfe-commits]

https://github.com/chenzheng1030 commented:

> 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.

For targets with ABI elfv2 which means there are dual entries, this patch can generate expected patchable entry:
For -mtriple=powerpc64-unknown-freebsd-unknown
```
.Ltmp0:
	nop
	nop
f3:                                     # @f3
.Lfunc_begin0:
	.cfi_startproc
.Lfunc_gep0:
	addis 2, 12, .TOC.-.Lfunc_gep0 at ha
	addi 2, 2, .TOC.-.Lfunc_gep0 at l
.Lfunc_lep0:
	.localentry	f3, .Lfunc_lep0-.Lfunc_gep0
# %bb.0:                                # %entry
	nop
```

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