build: Clean up build toolchain (and use musl for libc)

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.

.gitmodules

@@ -13,3 +13,6 @@
 [submodule "lib/EventFlow"]
 	path = lib/EventFlow
 	url = https://github.com/open-ead/EventFlow
+[submodule "toolchain/musl"]
+	path = toolchain/musl
+	url = https://github.com/open-ead/botw-lib-musl