[clang] [compiler-rt] [safestack] Various Solaris fixes (PR #98001)

[Rainer Orth via cfe-commits]

rorth wrote:

> They look quite independent. Would be possible to split independent fixed into a separate patches?

With the exception of the `-u` reordering, they are all Solaris-only, so I kept them together to avoid tons of micro-patches.  I certainly could split them if you prefer, something like
- `-u` reordering
- sparc enabling
- add `sanitizer_common` objects
- `pthread stacksize` quirk
- fix `TgKill`
- switch syscalls to `_REAL*`
- re-enable testing

Just let me know.

https://github.com/llvm/llvm-project/pull/98001