Eliminate libm-test.stmp.

math/Makefile uses libm-test.stmp to handle dependencies involving
multiple generated files all generated by a single sequence of
commands in a single Makefile rule.

Having separated the libm-test-ulps.h and libm-test.c generation into
separate runs of gen-libm-test.pl, there is now no need for a single
rule to generate multiple target files; each of the three target files
involved can be generated by a separate Makefile rule, meaning normal
dependencies on the individual files can be used and so libm-test.stmp
is not needed at all.  This patch does just that, eliminating the
.stmp file, in further preparation for when there are many separate
libm-test-<func>.c files generated from libm-test-<func>.inc and the
dependencies are on just the relevant .c file in each case.

Tested for x86_64.

	* math/Makefile (generated): Do not include libm-test.stmp.
	($(addprefix $(objpfx), $(libm-tests-generated))): Do not depend
	on $(objpfx)libm-test.stmp.
	($(objpfx)libm-test.stmp): Remove rule.
	($(objpfx)libm-test-ulps.h): New rule.
	($(objpfx)libm-test.c): Likewise.
	($(objpfx)libm-have-vector-test.h): Likewise.
	($(addprefix $(objpfx), $(libm-tests.o)): Depend directly on
	individual generated files, not libm-test.stmp.