ld: Add NOCF_PROTECTION_CFLAGS to turn off -fcf-protection

ld: Add NOCF_PROTECTION_CFLAGS to turn off -fcf-protection

GCC in Ubuntu 20.04 enables -fcf-protection by default, which leads to

FAIL: S-records
FAIL: S-records with constructors
FAIL: Build plt-main with -z bndplt
FAIL: Build plt-main with PIE and -z bndplt
FAIL: Build plt-main with -z bndplt -z now
FAIL: Build plt-main with PIE and -z bndplt -z now

on x86-64.  Add NOCF_PROTECTION_CFLAGS to pass -fcf-protection=none on
these tests.

	* testsuite/config/default.exp (NOCF_PROTECTION_CFLAGS): New.
	Set to "-fcf-protection=none" if target compiler supports it.
	* testsuite/ld-srec/srec.exp: Add $NOCF_PROTECTION_CFLAGS to
	CC and CXX.
	* testsuite/ld-x86-64/x86-64.exp: Add $NOCF_PROTECTION_CFLAGS
	to PLT BND tests.