This is useful for people who want to link using recent versions of
mold which are smart enough to e.g. optimise GOT loads; we need to
pass --no-relax to disable those optimisations in order to get matching
instructions.
Using musl is required to match several functions that use <math.h>
floating point functions like isnan.
This also removes a dependency on devkitA64. We previously only needed
it because Clang insisted on using GCC to link the final executable
owing to the use of a "bare metal" target like aarch64-none-elf.
We can make Clang invoke the linker itself by using a Linux target.
