[ARM] Don't pass -arm-execute-only to cc1as

The option -mexecute-only is translated into the backend option
-arm-execute-only. But this option only makes sense for the compiler and
the assembler does not recognize it. This patch stops clang from passing
this option to the assembler.

Change-Id: I4f4cb1162c13cfd50a0a36702a4ecab1bc0324ba
Review: https://reviews.llvm.org/D30414
llvm-svn: 296454