Pull up following revision(s) (requested by skrll in ticket #667):

sys/arch/aarch64/aarch64/sig_machdep.c: revision 1.9
sys/arch/aarch64/aarch64/cpu_machdep.c: revision 1.15

kern/58149: aarch64: Cannot return from a signal handler if SP was
misaligned when the signal arrived

Apply the kernel diff from the PR
1. sendsig_siginfo() previously assumed that user SP was always aligned to
    16 bytes and could call signal handlers with SP misaligned. This is a
    wrong assumption because aarch64 demands that SP is aligned *only while*
    it's being used to access memory. Now it properly aligns it before
    pusing anything on the stack.
2. cpu_mcontext_validate() used to check if _REG_SP was aligned and
    considered the ucontext invalid otherwise. This meant if a signal was
    sent to a process whose SP was misaligned, the signal handler would fail
    to return because the ucontext passed from the kernel was an invalid
    one. Now setcontext(2) doesn't complain about misaligned SP.

(martin)