* libm/common/s_log2.c: Change from using M_LOG2_E to M_LN2 define
(from math.h--the latter is POSIX, the former non-standard).
* libm/common/sf_log2.c: Ditto. Change cast for M_LN2 from float to
float_t (in case all math not done in float).
* libc/include/math.h: Ditto (same 2 things as sf_log2.c).