When grabbing registers for an lwp, fake the inferior's pid using the

When grabbing registers for an lwp, fake the inferior's pid using the
lwp ID before invoking the underlying target operation.

For corefiles, we rely on gdb internals to do this, and it uses the
pid as an index, rather than the lwpid, so previously, backtraces
for multithreaded core files wasn't working correctly. For processes,
we currently use ptrace directly, so fixup that code to also use
the pid directly.

Discussed With: marcel, davidxu
MFC After: 4 days