[Clang][Docs] Add help test to `-march` and `-mcpu` to suggest `-mcpu=help`

Currently there is no documentation for these flags, users might find it
confusing to know which values are permitted. This change simply adds
documentation to suggest using `-mcpu=help` to list the availible target
names.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D151098