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

configure

@@ -3734,6 +3734,10 @@ if enabled asm; then
 \n: .int 0
 .endm
 m x
+EOF
+
+    check_as <<EOF || $nogas "GNU assembler not found, install gas-preprocessor"
+.altmacro
 EOF
 
     check_as <<EOF && enable as_func