Define powerpc64 lroundl compat symbol only once.

Define powerpc64 lroundl compat symbol only once.

sysdeps/powerpc/powerpc64/fpu/multiarch/s_llround.c defines the
lroundl compat symbol, version GLIBC_2_1, twice, once based on llround
and once based on __lround.  Those are aliases for each other (llround
weak, __lround strong), but defining it twice does not make sense.
This patch changes it to define the compat symbol once only, matching
how libm_alias_double defines it.

Tested with build-many-glibcs.py for its powerpc64 configurations.

	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_llround.c
	[LONG_DOUBLE_COMPAT(libm, GLIBC_2_1)] (lroundl): Do not define
	compat symbol based on llround.