Base powerpc logbl compat symbols on __logb not logb.

Base powerpc logbl compat symbols on __logb not logb.

Some powerpc logb implementations define a compat symbol for logbl
based on logb, whereas libm_alias_double defines such a compat symbol
based on __logb instead.  This difference (logb is weak, __logb isn't)
is enough to result in different installed stripped shared libraries.
The difference in the installed libraries isn't significant, but first
changing the compat_symbol calls helps make it possible to validate a
subsequent change to use libm_alias_double by comparison of libraries,
so this patch does such a preliminary change.

Tested with build-many-glibcs.py for all its hard-float powerpc
configurations.

	* sysdeps/powerpc/power7/fpu/s_logb.c
	[LONG_DOUBLE_COMPAT (libm, GLIBC_2_0)] (logbl): Define as compat
	symbol based on __logb, not on logb.
	* sysdeps/powerpc/powerpc32/power4/fpu/multiarch/s_logb.c
	[LONG_DOUBLE_COMPAT (libm, GLIBC_2_0)] (logbl): Likewise.
	* sysdeps/powerpc/powerpc64/fpu/multiarch/s_logb.c
	[LONG_DOUBLE_COMPAT (libm, GLIBC_2_0)] (logbl): Likewise.