ARM64: Simplify save/restore regs in invoke stub.

Save/restore fewer registers and use common macros to do so.
Rewrite the return sequence to avoid many chained branches.
And a few other minor simplifications.

Test: Pixel 2 XL boots.
Test: testrunner.py --target --64 --optimizing
Change-Id: I32ee7bad685b8bd73d07e5a4c48a6ac0b22ff762
1 file changed