Make use of XCHG/BSWAP on x86, REV16 and REV on ARMv6-m and above,
and whatever other optimized instructions on other platforms.
Defines extra CONFIG variables, and removes the unused functions for
endian-swapping. Fixes some oversights in using the macros.
Squashed commit of the following:
libs/libxx: Fix some confusing in naming. If the directory is called libxx, then the library must be libxx.a (unless perhaps LIBCXX is selected).
libs/: Fix paths in moved library directories.
libs: Brute force move of libc, libnx, and libxx to libs. Cannot yet build it in that configuration.
