Made art/runtime/arch/stub_test.cc compile with -O2 again.

The test file art/runtime/arch/stub_test.cc wasn't compiling with -O2
as the optimisations interacted with -fstack-protector-strong. The
aarch64 _asm_ block in the Invoke3WithReferrerAndHidden function was
clobbering all possible registers, and clang requires at least one
register to be live across an _asm_ block to do the checking.

The fix was to remove a callee-saved register, x20, from the clobber
list of the aarch64 asm block of Invoke3WithReferrerAndHidden. The
block was also modified to save and restore x20 to ensure that it
won't be clobbered by the stubs invoked by the blr instruction. Also
added some comments above the clobber list.

Change-Id: I03597fd2d14cf2d6e32edf02835aee2eb68bab17