ISA: generic Linux thread info support

This patch takes the Linux thread info support scattered across
different ISA implementations (currently in ARM, ALPHA, and MIPS), and
unifies them into a single file.

Adds a few more helper functions to read out TGID, mm, etc.

ISA-specific information (e.g., ALPHA PCBB register) is now moved to
the corresponding isa_traits.hh files.
11 files changed