[Driver] Delete -mimplicit-it=
This is a GNU as and Clang cc1as option, not a GCC option.
Users should specify -Wa,-mimplicit-it= instead.
Note: mixing the -m option and the -Wa, option doesn't work
-Wa,-mimplicit-it=never -mimplicit-it=always =>
clang (LLVM option parsing): for the --arm-implicit-it option: may only occur zero or one times!
Reviewed By: nickdesaulniers, raj.khem
Differential Revision: https://reviews.llvm.org/D102568