CRT_degreeSign code shrink

[Explorer09]

Make CRT_degreeSign a writable buffer that will be updated after the initDegreeSign() function. This avoids needing a static buffer in the function.

Also use MB_LEN_MAX for the buffer size, which is more appropriate than the magic number 4.

One side effect of this change is that CRT_degreeSign will lose the const qualifier in the header (CRT.h).