math: don't clobber old libm.so on install [BZ #19822]

When installing glibc (w/mathvec enabled) in-place on a system with
a glibc w/out mathvec enabled, the install will clobber the existing
libm.so (e.g., /lib64/libm-2.21.so) with a linker script.  This is
because libm.so is a symlink to libm.so.6 which is a symlink to the
final libm-2.21.so file.  When the makefile writes the linker script
directly to libm.so, it gets clobbered.

The simple patch below to math/Makefile fixes this.  It is based on
the nptl Makefile, which does exactly the same thing in a safer way.