sys-libs/glibc: revert "use -mno-sdata, not -fcommon, bug #723268"

sys-libs/glibc: revert "use -mno-sdata, not -fcommon, bug #723268"

The underying linker failure cause is not just an overflow
but a functional difference in '_dl_sysinfo_break' symbol linkage.

'_dl_sysinfo_break' is defined as a function in assembly
but declared as a global 'common' variable. Making it non-common
actually breaks sysinfo handler which is expected to be called
without function descriptor indirection. As sysonfo handler
is called before rtld processed it's own relocations.

Let's revert back to -fcommon until we have better glibc fix.

This reverts commit 8ec720b67f38952a4b9c6054c6d8ef0fc79d0343.

Bug: https://bugs.gentoo.org/723268
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>