[PowerPC] Support various tls-related modifiers

The current code base only supports the minimum set of tls-related
relocations and @modifiers that are necessary to support compiler-
generated code.  This patch extends this to the full set defined
in the ABI (and supported by the GNU assembler) for the benefit
of the assembler parser.
[PowerPC] Support R_PPC_REL16 family of relocations

The GNU assembler supports (as extension to the ABI) use of PC-relative
relocations in half16 fields, which allows writing code like:

  li 1, base-.

This patch adds support for those relocation types in the assembler.