[clang] [PowerPC] Support local-dynamic TLS relocation on AIX (PR #66316)

[Felix via cfe-commits]

orcguru wrote:

> The primary advantage of the local-dynamic access method is that you only make a single function call to __tls_get_mod() and use the returned pointer to get at all TLS variables. This implementation makes a function call for every variable. The only thing you've gained is a reduction in the number of TOC symbols.

The issue with duplicated calls to .__tls_get_mod is due to those pseudo TLSLDAIX nodes depending on different GVs, and CSE is not able to do dedup. I probably have to create a pseudo GV named like "_$TLSML", and use that GV for all TLSLDAIX pseudo nodes.

https://github.com/llvm/llvm-project/pull/66316