Add MIPS support to the coredump library. Explicit support for the MIPS
program counter register is added so that we can start backtracing from
the program counter value we read from a core dump. The PC register
support was not strictly required for local backtracing, and we will in
fact just plug the return address to the PC register in getcontext().
I have only tested the 32bit "OABI" paths.
Dropping the extra frame for unw_backtrace itself using unw_step is
approximately 15% slower than skipping the frame in tdep_trace. So
drop the frame in the latter, and make the function a private
implementation detail for libunwind, not an exported interface.
Also moves unw_getcontext call back into unw_backtrace to avoid an
extra call frame in case slow_backtrace does not get inlined into
unw_backtrace.
Adds new function to perform a pure stack walk without unwinding,
functionally similar to backtrace() but accelerated by an address
attribute cache the caller maintains across calls.
