x86_64-cygwin tests

x86_64-cygwin tests

Tests just having "xfail: x86_64-*-cygwin" aren't good, since
presumably if a test fails on x86_64-cygwin then it also fails on
x86_64-*-pe* and x86_64-*-mingw*.

binutils/
	* testsuite/lib/binutils-common.exp (is_pecoff_format): Accept
	optional machine-os arg.
ld/
	* testsuite/ld-scripts/default-script1.d: Don't skip, xfail
	using is_pecoff_format.
	* testsuite/ld-scripts/default-script2.d: Likewise.
	* testsuite/ld-scripts/default-script3.d: Likewise.
	* testsuite/ld-scripts/default-script4.d: Likewise.
	* testsuite/ld-scripts/pr20302.d: Remove x86_64-*-cygwin from notarget.
	* testsuite/ld-scripts/provide-6.d: Remove x86_64-*-cygwin from xfail.
	* testsuite/ld-scripts/provide-8.d: Likewise.