binutils: backport xtensa ld optimizations

This series optimizes most time-consuming algorithms and data structures
in the xtensa link-time relaxation code, leaving relaxation logic intact.

Speedup linking typical linux kernel is ~8 times (1 minute instead of 8),
pathological cases (linking objects partially linked without relaxation)
are handled ~60 times faster (1 minute instead of an hour).

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>