gold: Use char16_t, char32_t instead of uint16_t, uint32_t as character types

The std::basic_string template type is only specified for
instantiations using character types.  Newer (LLVM) libc++
implementations no longer allow non-character integer types
to be used.

gold/
	* output.cc: Include <uchar.h>.
	(Output_section::add_merge_input_section): Use char16_t and
	char32_t for 2- and 4-byte entry size, respectively.
	* stringpool.cc: Include <uchar.h>.
	(Stringpool_template): Explicitly instantiate for char16_t,
	char32_t instead of uint16_t, uint32_t.
	* merge.cc (Output_merge_string): Likewise.