Use parse_integer to avoid sign extension.

Coverity warned about gdb_write_mem sign extending the result of
parse_byte shifted left by 24 bits when generating a 32-bit memory
write value for MMIO. Simplify the code by using parse_integer
instead of unrolled parse_byte calls.

CID: 1401600
Reviewed by: cem
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D20508