Allow Clang's integrated assembler to assemble boot0

Allow Clang's integrated assembler to assemble boot0

dim@ compared clang IAS-built and GNU as-built boot0 and found them
equivalent.  IAS encoded one instruction using two bytes where GNU as
used three, and another instruction using three bytes where GNU as used
two.  The net result is equivalent and tested, so there is no need to
force IAS off for boot0.