memmap.cpp: At startup, don't initialise memory that is initialised when loading a ROM.

Based on <https://github.com/ShadauxCat/CATSFC/commit/c9c91f327a4ded2f52f13c449690a334623e0fdd> by Nebuleon.
memmap.cpp: Comment fix.

64 kilobytes is not equivalent to 512 megabits, and 128 kilobytes is not equivalent to 1024 megabits.
Allow ports to specify if they unconditionally enable or disable SupportHiRes.
Allow ports to specify if they unconditionally enable or disable SixteenBitSound.
Allow ports to specify if they unconditionally enable or disable ReverseStereo.
Allow ports to specify if they unconditionally enable or disable [Disable]GraphicWindows.
Allow ports to specify if they unconditionally enable or disable Stereo.