[clang] [PAC][clang][Driver] Add signed GOT flag (PR #96160)

[Anton Korobeynikov via cfe-commits]

asl wrote:

> Do we want a lot of `-fptrauth-xxx` instead of `-fptrauth-something=xxx,yyy,zzz`?

I would lean towards a single flag. However, I do not know how this would affect Apple downstream and what are preferences there. 

Tagging @ahmedbougacha @ahatanak @ojhunt 

https://github.com/llvm/llvm-project/pull/96160