- Multiboot support added back in FreeLdr MSVC builds so that it can be
booted using e.g. GRUB. See CORE-15563.
- Re-introduce the disk drive mapper code.
As the MSVC linker alone doesn't permit such control, the file uses
ASM language (C can alternatively be used), together with extra linker
command-line switches.
It is pre-processed 3 times: first for generating the ASM code, second
for the C code and the third time for generating the linker response
file.
In our case, the ASM code defines the __bss_start__ and __bss_end__
symbols that allow us to find the limits of the .bss section (which is
by the way automatically appended to the .data section by the MSVC linker).
The C code is used to specify the list of linker switches that can be
passed through the `#pragma comment(linker, ...)' directive (the can be
alternatively specified in the linker response section).
Finally the linker response section contains all the linker switches
that cannot be specified with the `#pragma comment(linker, ...)'
directive.
Using all this we can recycle the BSS initialization code, that has been
written originally for GCC only, also for the MSVC builds.
Also, remove the outdated .text16 section merging.
