toolchain-glibc.eclass: fix libm.so symlinking for live glibc

toolchain-glibc.eclass: fix libm.so symlinking for live glibc

The failure happens when live glibc-9999 ebuild is installed:
 * QA Notice: Missing gen_usr_ldscript for libm-2.26.90.so
 * ERROR: sys-libs/glibc-9999::gentoo failed:
 *   add those ldscripts

The problem here is how upstream glibc version is detected:
    dosym ../../$(get_libdir)/libm-${PV}.so $(alt_usrlibdir)/libm-${PV}.so

Change to use 'version.h' to pick upstream version.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Signed-off-by: Matthias Maier <tamiko@gentoo.org>