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: Martin Storsjö <martin@martin.st>
