[libc++] Fix incorrect usage of __STDC_HOSTED__

D56913 introduced the _LIBCPP_FREESTANDING macro and guarded its
definition by:

	#ifndef __STDC_HOSTED__
	#  define _LIBCPP_FREESTANDING
	#endif

However, __STDC_HOSTED__ is defined as 0 in freestanding implementations
instead of undefined, which means that _LIBCPP_FREESTANDING would never
get defined. This patch corrects the above as:

	#if __STDC_HOSTED__ == 0
	#  define _LIBCPP_FREESTANDING
	#endif

Differential Revision: https://reviews.llvm.org/D86055