This patch allows clang users to print out a list of supported CPU models using
clang [--target=<target triple>] --print-supported-cpus
Then, users can select the CPU model to compile to using
clang --target=<triple> -mcpu=<model> a.c
It is a handy feature to help cross compilation.
While the flag seems to be a front-end option, the supported CPU models are stored in each target machine in the back-end. This patch attempts to create a short cut to directly retrieve information from the back-end target machines at the front-end.