Provide log2 and nexttoward implementations.

log2 was taken from upstream bionic's msun folder (as of today). The
only local change is the definition of __weak_reference in e_log2.c.

nexttoward can be implemented in terms of nextafter, since long double
is the same as double on android.

Fixes libc++'s depr/depr.c.headers test.

Change-Id: I2bdc23795062d72b47efc96147055db22eed97a0