Allow case-insensitive values for -mcpu for AArch64 target in line with GCC.

GCC allows case-insensitive values for -mcpu, -march and -mtune options.
This patch implements the same behaviour for the -mcpu option for the AArch64 target.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@239619 91177308-0d34-0410-b5e6-96231b3b80d8