Expose an additional 16 KB of RAM in TWL mode by moving DTCM around a bit

[asiekierka]

If we observe the memory map:

0x2FF0000: libnds shadows this area with DTCM, making it inaccessible through this address.
0x2FF4000: devkitARM bootstub structure

The entire area of memory from 0x2FF4000 to 0x2FFFFFF is effectively reserved, either for the devkitARM bootstub header or by data pre-loaded by the NTR/TWL loader. However, the area of main RAM from 0x2FF0000 - 0x2FF3FFF is shadowed and effectively unused; this doesn't matter on NTR consoles, as it's just a mirror of the 4/8 MB of RAM, but on TWL this constitutes an additional 16KB of RAM that could still be used for heap/stack data.

There's three identified changes that have to be done to allow for this:

• move __dtcm_start to 0x2FF4000 in the link script (mpu_setup.s uses the symbol anyway);
• make sure _exit() disables DTCM whenever it accesses bootstub data - this would probably require porting the necessary code to ASM routines to guarantee it doesn't use the stack, which is in DTCM;
• (optional) patch the FAT driver's IS_MAIN_RAM macro to use the same.

Note that this is a low priority change; 16KB relative to (almost) 16MB is almost unnoticeable.