x86-64: Limit PIC check to shared library build

x86-64: Limit PIC check to shared library build

When building an executable, undefined symbols are error and undefined
weak symbols are resolved to zero.  We only need to check PIC for
building a shared library.

bfd/

	PR ld/21782
	* elf64-x86-64.c (elf_x86_64_relocate_section): Limit PIC check
	to shared library.

ld/

	PR ld/21782
	* testsuite/ld-x86-64/pie3-nacl.d: New file.
	* testsuite/ld-x86-64/pie3.d: Likewise.
	* testsuite/ld-x86-64/pie3.s: Likewise.
	* testsuite/ld-x86-64/x86-64.exp: Run pie3 and pie3-nacl.