Create ptov() function.

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.

(cherry picked from commit ed2a65769a5d04fcfc2acff3fa11d6b69394fd88)