This is useful for the RecordWrite stub that can now specify the set
of allocatable registers in its call descriptor interface. 
During register allocation a custom register configuration is used to
ensure that the register are allocated from the given set.

This makes calling RecordWrite stub less expensive as we need to save/restore
only the allocatable registers instead all registers.

Bug: chromium:749486
Change-Id: If4d73f1fd525e480970ea92600fb811e63677eb5
Reviewed-on: https://chromium-review.googlesource.com/624734Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Albert Mingkun Yang <albertnetymk@google.com>
Cr-Commit-Position: refs/heads/master@{#47577}