Create ptov() function.

Create a ptov() function. It's basically the same as the btx PTOV
macro, but works everywhere. smbios needs this to translate addresses,
but the translation differs between BIOS booting and EFI booting. Make
it a function so one smbios.o can be used everywhere. Provide
definitions for it in the two loaders affected.

Move smbios.c to libsa.

smbios used to be an i386 only kinda weird quirk to the x86
architecture. But UEFI picked it up, dusted it off and now it's many
other locations. Make it base technology by moving it to libsa and
fixing up the compliation. The code has issues with unaligned access
still, but that will be addressed in a followup commit.