MFC r361024:

[PowerPC] Fix wrong instructions in _savegpr_X.

We were accidentally using stfd instead of stw in our SAVEGPR macro.

This has almost certainly been causing crashes when compiling with -Os.

Reviewed by: jhibbits (in irc)
Sponsored by: Tag1 Consulting, Inc.

(cherry picked from commit 3dbb9df36bdb0a7e8e00f24f94fa06a802a3eeee)