arch(7): PowerPC64LE architecture definition

Document the new powerpc64le arch's initial specifications.

Certain things are subject to change while this is experimental. The most
likely change is that long double may switch to quad, dependent on POWER8
emulation assistance for __float128 being set up in the compiler (as
POWER8 does not have IEEE-compatible 128-bit hardware float, unlike POWER9.)

Sponsored by: Tag1 Consulting, Inc.