x86_64: fix static build of __mempcpy_chk for compilers defaulting to PIC/PIE

When glibc is compiled with gcc 6.2 that has been configured with
to default to PIC/PIE, the static version of __mempcpy_chk is not built,
as the test is done on PIC instead of SHARED.  Fix the test to check for
SHARED, like it is done for similar functions like __memcpy_chk.

2017-03-12  Mike Frysinger  <vapier@gentoo.org>

	* sysdeps/x86_64/mempcpy_chk.S (__mempcpy_chk): Check for SHARED
	instead of PIC.