gas: microblaze: fix shift overflow

gas: microblaze: fix shift overflow

This code tries to shift an integer 31 bits which triggers a werror:
gas/config/tc-microblaze.c:742:21: error: integer overflow in expression [-Werror=overflow]
  e->X_add_number |= -(1 << 31);

Cast the 1 to offsetT to match X_add_number to fix things.