crt: Don't initialize lconv to unsigned char mode

Since the initial commit of mingw-w64, there's a call (in a
constructor in crt/charmax.c, pulled in via the mingw_initcharmax
variable) to __lconv_init(), which reconfigures the CRT's lconv
info to unsigned char mode - but there's no explanation for why
this is the case.

The fact that mingw-w64 initializes lconv info this way breaks
a test in llvm's libc++ (where it expects lconv's frac_digits to
be intialized to CHAR_MAX, not UCHAR_MAX). This also differs to
how MSVC environments behave, where they are initialized to
CHAR_MAX.

So unless there's a clear known reason why we'd initialize it
to unsigned char mode, revert to the same behaviour as MSVC has.

Signed-off-by: Martin Storsjö <martin@martin.st>