x86_64: fix static build of __memcpy_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 __memcpy_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 memmove_chk.

Changelog:
	* sysdeps/x86_64/memcpy_chk.S (__memcpy_chk): Check for SHARED
	instead of PIC.