ARM64: Improve code generated to spill/restore for slow paths.

Aligning the accesses allows generating better code.

Before:

    add x16, sp, #0x44 (68)
    stp x0, x1, [x16, #-16]

After:

    stp x0, x1, [sp, #56]

Change-Id: I3e20ad3fa59d00aee4b4d14ea9d59c7cd546509e
3 files changed