C code or compiler built-ins are preferable over inline assembler for
byte-swaps as it allows for better optimisations (e.g. instruction
scheduling) which would otherwise be impossible.

As with f64c2e71 for x86 and Arm,
this removes the inline assembler on GCC (and Clang) since we now
require recent enough compiler versions. This indeed seems to work on
AArch64, SuperH and, if Zbb is enabled, RISC-V. (AVR32 was not tested
since it has no known working compilers at this time.)