[AIX][TLS] Add ASM portion changes to support TLSGD relocations to XCOFF objects

- Add new variantKinds for the symbol's variable offset and region handle
- Print the proper relocation specifier @gd in the asm streamer when emitting
  the TC Entry for the variable offset for the symbol
- Fix the switch section failure between the TC Entry of variable offset and
  region handle
- Put .__tls_get_addr symbol in the ProgramCodeSects with XTY_ER property

Reviewed by: sfertile

Differential Revision: https://reviews.llvm.org/D100956