Add GLIBC_2.3 to libm in Versions.def

This patch adds GLIBC_2.3 mark on libm so it is always
define in abi-versions.h. This fixes a build issue with
fe_nomask.c in PPC64 LE where GLIBC_2_3 are no defined
for SHLIB_COMPAT, resulting in a wrong evaluation in the macro.
Fix wrong ChangeLog merge