Store the MSVC compiler target architecture ('x86', 'x64' or 'ARM' (this
is ARM64, I believe)), rather than just if it's x64 or not.
The regex used for target architecture should be ok, based on this list
of [1] version outputs, but we assume x86 if no match, for safety's
sake.
[1] https://stackoverflow.com/a/1233332/1951600
Also detect arch even if cl outputs version to stdout.
Ditto for clang-cl
Future work: is_64 is now only used in get_instruction_set_args()