bsd.cpu.mk: Introduce MACHINE_ABI

MACHINE_ABI is a list of properties of the ABI used for MACHINE_ARCH.
It should be used in place of long conditionals on MACHINE_ARCH where
practical.

The following properties are indicated with one of the follow values:

Byte order: big-endian, little-endian
Floating point ABI: soft-float, hard-float
Size of long (size_t, etc): long32, long64
Pointer type: ptr32, ptr64
Size of time_t: time32, time64

For example, i386 targets will be:
MACHINE_ABI= big-endian hard-float long32 ptr32 time32

Reviewed by: imp
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D36421