Clean up libm vector tests exception test disabling.

The libm vector tests disable tests of exception raising via defining
macros EXCEPTION_TESTS_float and EXCEPTION_TESTS_double to 0 in the
headers for individual vector lengths.

As EXCEPTION_TESTS is used in code in libm-test-driver.c that is
otherwise ready to be built only once per type, this is not a good
idea; it's better to define TEST_EXCEPTIONS appropriately so that
flag_test_exceptions then gets initialized appropriately.
Furthermore, it's better to do this just once, in test-math-vector.h,
since there is no actual dependence on the vector length or type.
This patch duly makes that change.

Tested for x86_64.

	* math/test-math-finite.h (TEST_EXCEPTIONS): New macro.
	* math/test-math-no-finite.h (TEST_EXCEPTIONS): Likewise.
	* math/test-math-vector.h (TEST_EXCEPTIONS): Likewise.
	* math/test-math-no-inline.h (TEST_EXCEPTIONS): Remove macro.
	* math/test-double-vlen2.h (EXCEPTION_TESTS_double): Likewise.
	* math/test-double-vlen4.h (EXCEPTION_TESTS_double): Likewise.
	* math/test-double-vlen8.h (EXCEPTION_TESTS_double): Likewise.
	* math/test-float-vlen4.h (EXCEPTION_TESTS_float): Likewise.
	* math/test-float-vlen8.h (EXCEPTION_TESTS_float): Likewise.
	* math/test-float-vlen16.h (EXCEPTION_TESTS_float): Likewise.