Rename MACHINE_ABI and TARGET_ABI

The MACHINE_ABI and TARGET_ABI variables are used to set the middle of
the target triple (e.g., "-unknown-" or "-gnueabihf-"). They are not set
by any tool in the base system and I've only found the latter mentioned
in one review online. As such, rename them to to MACHINE_TRIPLE_ABI and
TARGET_TRIPLE_ABI to clear the way to use MACHINE_ABI as a supplement to
MACHINE_CPU, etc.

Reviewed by: imp, jhb
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D36420