Mark internal wchar functions with attribute_hidden [BZ #18822]

Mark internal wchar functions with attribute_hidden to allow direct
access within libc.so and libc.a without using GOT nor PLT.

	[BZ #18822]
	* include/wchar.h (__wcsnlen): Add attribute_hidden.
	(__wcscat): Likewise.
	(__btowc): Likewise.
	(__wcrtomb): Likewise.
	(__mbsrtowcs): Likewise.
	(__wcsrtombs): Likewise.
	(__mbsnrtowcs): Likewise.
	(__wcsnrtombs): Likewise.
	(__wcsncpy): Likewise.
	(__wcpncpy): Likewise.
	(__wmemcpy): Likewise.
	(__wmempcpy): Likewise.
	(__wmemmove): Likewise.
	(__wcschrnul): Likewise.
	(__vfwscanf): Likewise.
	(__vswprintf): Likewise.
	(__fwprintf): Likewise.
	(__vfwprintf): Likewise.