scripts: create CT_HEADERS_DIR

"${CT_SYSROOT_DIR}/usr/include" is only for "${CT_USE_SYSROOT}" = "y".
We should also mkdir when "${CT_USE_SYSROOT}" != "y".
"${CT_HEADERS_DIR}" can support both cases.

Signed-off-by: Zhenqiang Chen <zhenqiang.chen@linaro.org>
(transplanted from bdff4164a1227ebdd538e0ad3c9b6862b3c5a253)

 --- gcc-4.3.4/libffi/src/arm/sysv.S	

 +++ gcc-4.3.4/libffi/src/arm/sysv.S	

 @@ -235,7 +235,7 @@ ARM_FUNC_START ffi_closure_SYSV

  	stmfd	sp!, {ip, lr}

  	UNWIND .save	{r0, lr}

  	add	r2, sp, #8

 -	.pad #16

 +	UNWIND .pad #16

  	sub	sp, sp, #16

  	str	sp, [sp, #8]

  	add	r1, sp, #8