Makefile improvements

[tommai78101]

Changes:

• Provided a better way to build relevant NTR payloads, depending on which targeted platform to build for (old_3ds, new_3ds, or both).
• For old_3ds, made specific changes to build DSP using -O3, and use -Os for the rest.

[tommai78101]

On second thought, this Makefile improvement isn't really improving the build flow. What it does right now is, it runs Make twice, overriding the NEW_3DS flag set in the previous Make run, thus ignoring the DSP path to be compiled using -Os. I may need to rethink this at a later time.

[tommai78101]

Going by WinterMute's suggestion, I added #pragma GCC optimize ("O3") to source/dsp source file, so it should theoretically override the flags set in the Makefile, and will compile with -O3 at all times for source codes marked with the #pragma GCC call. It is theoretical, because I'm not well-versed with #pragma GCC calls inside source codes.

@44670, what do you think about the #pragma GCC calls?