[clang][Arm] Fix handling of -Wa,-implicit-it=

Similiar to D95872, this flag can be set for the assembler directly.
Move validation code into a reusable helper function.

Link: https://bugs.llvm.org/show_bug.cgi?id=49023
Link: https://github.com/ClangBuiltLinux/linux/issues/1270


Reported-by: Arnd Bergmann <arnd@kernel.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D96285