configure: clean up check_inline_asm and check_as functions

The check_inline_asm function should check the actual C compiler,
not the one used for assembly files.  Usually these are the same,
but they might be different, typically when using a compiler other
than gcc.

The check_as should, as its name suggests, test the type of input
the AS command is used with, i.e. a standalond assembly (.S) file.

Finally, check for gnu assembler using the modified check_as as
this reflects actual usage.

Signed-off-by: Mans Rullgard <mans@mansr.com>
