While char is signed on macOS AArch64, on Linux it is unsigned. This
means it cannot represent the -1 return value of getopt_long.
Change the type of `c` to int, the type that getopt_long returns, so
that the -1 will be kept intact and can be checked for.
Fixes: c6be4f85e3 ("pan/bi: Use getopt for bifrost_compiler")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12153>
Icecream95
490d09f1a4