configure: use .altmacro for gnu as check on arm

Clang's integrated assembler (after 3.4) does not yet support the
'.altmacro' directive which is only used in arm asm. Support is planned:
http://llvm.org/bugs/show_bug.cgi?id=18918
Merge commit '7b06ddb8352fe7f434414d7911ced94956bb25b2'

* commit '7b06ddb8352fe7f434414d7911ced94956bb25b2':
  configure: use .altmacro for gnu as check on arm

Merged-by: Michael Niedermayer <michaelni@gmx.at>