configure: arm: Assume softfp ABI on darwin

Don't try to detect the float ABI by checking at the toolchain
name or by trying to assemble and link files with eabi_attributes.

This fixes the float ABI detection when building using clang
with -fembed-bitcode, where the current eabi_attributes check
accidentally passes.

This issue was pointed out by James Howe <james.howe@hp.com>.

CC: libav-stable@libav.org
Signed-off-by: NMartin Storsjö <martin@martin.st>

configure

@@ -4015,7 +4015,7 @@ elif enabled arm; then
         enable vfp_args
     elif check_cpp_condition stddef.h "defined _M_ARM_FP && _M_ARM_FP >= 30"; then
         enable vfp_args
-    elif ! check_cpp_condition stddef.h "defined __ARM_PCS || defined __SOFTFP__"; then
+    elif ! check_cpp_condition stddef.h "defined __ARM_PCS || defined __SOFTFP__" && [ $target_os != darwin ]; then
         case "${cross_prefix:-$cc}" in
             *hardfloat*)         enable vfp_args;   fpabi=vfp ;;
             *) check_ld <<EOF && enable vfp_args && fpabi=vfp || fpabi=soft ;;