[bios stage1] Delocalised string functions.
[bios stage1] Add itoa.
[bios stage1] Make strlen use %cx.
[bios stage1] Remove extraneous {push,pop}al and save flags across a20 init.
[bios stage1] Make A20 enabling code check from BIOS whether fast A20 is supported.
[bios stage1] Fix segment initialization order in init.