pcem: make MGA high-bit masks unsigned

pcem: make MGA high-bit masks unsigned

MGA defines several bit-31 masks from a signed int literal.
Shifting a signed 1 into the sign bit is undefined behavior in C.

Use UINT32_C(1) so the masks are built in unsigned arithmetic.
This matches the same cleanup merged upstream in 86Box, with the
extra OPTION_POWERPC mask that exists in this imported copy.