Switch to using the .code16 as(1) directive and using 16-bit assembly

Switch to using the .code16 as(1) directive and using 16-bit assembly
code instead of using 32-bit code and having to just "know" that it's
really 16-bit instructions when things run.  This also allows the code
to use fewer macros and more actual assembly statements, which eases
maintenance.  Unfortunately, due to as(1) brokenness, we still use m4
macros for all 16-bit addresses, and all short jumps (i.e., 8-bit
relative addresses in the jump instruction) must be wrapped in .code32
directives to avoid useless bloat by as(1).  This also fixes a few
problems that were preventing boot0 from compiling with the latest
and greatest version of as(1).