Apply fix for clang crashing on invalid -Wa,-march= values

Apply fix for clang crashing on invalid -Wa,-march= values

Merge commit df08b2fe8b35 from llvm git (by Dimitry Andric):

  [AArch64] Avoid crashing on invalid -Wa,-march= values

  As reported in https://bugs.freebsd.org/260078, the gnutls Makefiles
  pass -Wa,-march=all to compile a number of assembly files. Clang does
  not support this -march value, but because of a mistake in handling
  the arguments, an unitialized Arg pointer is dereferenced, which can
  cause a segfault.

  Work around this by adding a check if the local WaMArch variable is
  initialized, and if so, using its value in the diagnostic message.

  Reviewed By: tschuett

  Differential Revision: https://reviews.llvm.org/D114677

PR:		260078
Reported by:	bz
MFC after:	3 days

(cherry picked from commit a9cd5c30d64e213c537c76c2a261f7a222f348bf)