mbr.S: rewrite to use Intel syntax

[jessicah]

In order to be able to write the MBR boot code when creating an Intel partition map on non-x86 Haiku installations, we first need to rewrite it to Intel syntax so that we can use NASM to compile it, as the gcc assembler won't understand x86 assembly for non-x86 builds (e.g. an ARM build).

The MBR boot code was previously written in Intel syntax and compiled with NASM; however, this was changed in hrev49240 in order to implement #4028.