x86: change ap_boot_mtx from spinlock mutex to naive lock

[kib]

Summary

Problem is that the printfs done under the spinlock might be very slow,
then spinlock triggers the panic due to some AP holding the mutex too
long.  We do want to ensure that the printfs are serialized, still.

Also, reduce the scope of the lock. The local APIC and MTRR
initializations are local to the AP, while mca_init() protects the
shared state with mca-private spinlock.

PR:     289297

[markj]

sys/x86/x86/mp_x86.c

1099

[kib]

Fix grammar in the comment.
Use PAUSE if CAS failed.