x86 vdso: fix build locale dependency

Priit Laes discovered that the sed command processing nm output was
sensitive to locale settings.  This was addressed in commit
03994f01

 by using [:alnum:] in place of
[a-zA-Z0-9].

But that solution too is locale-dependent and may not always match
the identifiers it needs to.  The better fix is just to run sed et al
with a fixed locale setting in all builds.

Signed-off-by: Roland McGrath <roland@redhat.com>
CC: Priit Laes <plaes@plaes.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>