smm: Replace SMI assembler code with C code.

Convert the SMI handler from assembly to C.  This makes the handler
easier to understand and enhance.

The new handler will use references to the reserved memory at
0xf0000-0x100000.  If the physical memory in that range is modified at
runtime, then the SMI handler will cease to function properly (and may
allow unintended code to run in SMM mode).  However, that area is
marked as reserved and is normally made read-only at runtime, so there
is little risk in relying on it.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>