test/math: add fallback to libm-test-ulps-generic logic

Under test/math/, there is libm-test-ulps-generic used as fallback
in case that libm-test-ulps-$(TARGET_ARCH) does not exist.

This is required for arches that don't have ULPS yet.

Original patch from Jian Peng was found here:
http://lists.uclibc.org/pipermail/uclibc/2011-April/045172.html

Signed-off-by: Jian Peng <jipeng2005@gmail.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Anton Kolesov <akolesov@synopsys.com>