crt/stddef.bi: Use signed 32bit wchar_t on Linux/BSD

At least on GNU/Linux, wchar_t is signed 32bit, not unsigned. Of course
FB's wstring is still a UInteger, but the CRT headers may aswell use the
right type for C bindings etc.
crt/win32/stdio.bi: Fix Win64 stdin/stdout/stderr declarations

(syntax error, mistake in 0642043024df93205ccfbe63d77f3844ec6eb938)
crt/stdint.bi: Fix WCHAR_MIN/MAX declarations

As a side-effect (besides pulling in WCHAR_MIN/MAX), the addition of
	#include once "crt/stdint.bi"
to crt/win32/wchar.bi pulls in intptr_t which is needed there since
0642043024df93205ccfbe63d77f3844ec6eb938 and allows the header to compile
again.
tests: Fix crt/varargs.bas to pass 32bit Longs where snprintf() wants ints