[SystemZ] Support all TLS access models - CodeGen part
The current SystemZ back-end only supports the local-exec TLS access model.
This patch adds all required CodeGen support for the other TLS models, which
means in particular:
- Expand initial-exec TLS accesses by loading TLS offsets from the GOT using @indntpoff relocations.
- Expand general-dynamic and local-dynamic accesses by generating the appropriate calls to __tls_get_offset. Note that this routine has a non-standard ABI and requires loading the GOT pointer into %r12, so the patch also adds support for the GLOBAL_OFFSET_TABLE ISD node.
- Add a new platform-specific optimization pass to remove redundant __tls_get_offset calls in the local-dynamic model (modeled after the corresponding X86 pass).
- Add test cases verifying all access models and optimizations.