x86-64: Pass -z separate-code to ld for -z mark-plt tests

Pass -z separate-code to ld for -z mark-plt tests to fix:

FAIL: ld-x86-64/mark-plt-1a
FAIL: ld-x86-64/mark-plt-1b
FAIL: ld-x86-64/mark-plt-1c
FAIL: ld-x86-64/mark-plt-1d
FAIL: ld-x86-64/mark-plt-1a-x32
FAIL: ld-x86-64/mark-plt-1b-x32
FAIL: ld-x86-64/mark-plt-1c-x32
FAIL: ld-x86-64/mark-plt-1d-x32

when binutils is configured with --disable-separate-code.

	* ld-x86-64/mark-plt-1a-x32.d: Pass -z separate-code to ld.
	* ld-x86-64/mark-plt-1a.d: Likewise.
	* ld-x86-64/mark-plt-1b-x32.d: Likewise.
	* ld-x86-64/mark-plt-1b.d: Likewise.
	* ld-x86-64/mark-plt-1c-x32.d: Likewise.
	* ld-x86-64/mark-plt-1c.d: Likewise.
	* ld-x86-64/mark-plt-1d-x32.d: Likewise.
	* ld-x86-64/mark-plt-1d.d: Likewise.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>