Implement 64-bit atomic ops for ARMv6K+.

This implements the "quasi-atomic" 64-bit operations using LDREXD/STREXD
when those instructions are available (e.g. our ARMv7-A devices).

Also, folded in an expanded version of AtomicSpeed.c that has been
floating around my hard drive for a while.

Change-Id: I2d10c60ce2bce846218c3ac6dc22a54bc6627919
4 files changed