[PATCH] [AArch64] Implement Clang CLI interface proposal about "-march".

[Eric Christopher]

>> 3. "-march" is designed to be used only if user wants to use crc and crypto instructions, or disable fp/neon. So "-march" will not be frequently used and won't bring too much finger burden.
>
> I thought the idea was to encourage -march... at least on new targets...
>

Correct and aarch64 is a new target.

-march should be the way that people choose things. We can have -mcpu
be a copy of -march if we want, but -march is the command line option
that should be used.

-eric