MINOR: [C++][Gandiva] cast to unsigned char before ctype calls

[metsw24-max]

Out-of-range argument passed to ctype functions

The Gandiva string and time helpers feed raw char bytes into isdigit/isxdigit/isalpha/toupper. Where char is signed, any byte above 0x7f (so any multi-byte UTF-8 value in untrusted column data) is sign-extended to a negative int, which sits outside the unsigned char/EOF domain those functions are defined for. glibc tolerates it, but a strict ctype table indexes before its lookup array. Cast each argument to unsigned char at the call site, the way gdv_function_stubs.cc already does. Reviewer note: this is the same swept across both files so no instance is left behind.