Handle narrowing function sNaN test disabling based on argument format.

Testing narrowing functions for x86_64 with GCC 6 showed up a further
testsuite fix needed: there is no _Float128 sNaN support before GCC 7
on x86_64 / x86, and the existing tests of SNAN_TESTS only checked it
for the return type, not for the argument type.  This patch fixes the
code to check SNAN_TESTS (ARG_FLOAT) as well (in a variable set in
libm-test-driver.c, since libm-test-support.c is compiled only once
for each choice of FLOAT).

Tested for x86_64 and x86 with GCC 6 in conjunction with the main
patch adding narrowing add functions.

	* math/libm-test-driver.c (snan_tests_arg): New variable.
	* math/libm-test-support.h (snan_tests_arg): New declaration.
	* math/libm-test-support.c (enable_test): Check snan_tests_arg.