Preserve SSE registers in runtime relocations on x86-64.

SSE registers are used for passing parameters and must be preserved
in runtime relocations.  This is inside ld.so enforced through the
tests in tst-xmmymm.sh.  But the malloc routines used after startup
come from libc.so and can be arbitrarily complex.  It's overkill
to save the SSE registers all the time because of that.  These calls
are rare.  Instead we save them on demand.  The new infrastructure
put in place in this patch makes this possible and efficient.

stdio-common/scanf15.c

@@ -1,5 +1,6 @@
 #undef _GNU_SOURCE
 #define _XOPEN_SOURCE 600
+#undef _LIBC
 /* The following macro definitions are a hack.  They word around disabling
    the GNU extension while still using a few internal headers.  */
 #define u_char unsigned char