libstdc++: Make <cstdint> work freestanding [PR107134]

libstdc++: Make <cstdint> work freestanding [PR107134]

When gcc/config.gcc defines use_gcc_stdin=wrap, GCC's <stdint.h> tries
to use libc's <stdint.h> unless -ffreestanding is used.

When libstdc++ is configured --disable-hosted-libstdcxx we want
<cstdint> to work even without -ffreestanding being given. This is a
kluge to make it include GCC's <stdint-gcc.h> directly even without
-ffreestanding.

libstdc++-v3/ChangeLog:

	PR libstdc++/107134
	* include/c_global/cstdint [!_GLIBCXX_HOSTED]: Include
	<stdint-gcc.h> directly.