Adds builds of newlib-3.3.0, and implement an allocator in n64lib which
uses this.

Newlib was configured as follows:

    CFLAGS_FOR_TARGET="-DPREFER_SIZE_OVER_SPEED=1 -Os -G0 -mabi=32 -mno-gpopt" \
        ../configure --target=mips-elf

libc.a and libm.a are bundled in the cargo-n64 executable, and written
to the temp directory where the other temporary build artifacts are
placed (linker script, for example).

Since our sbrk() starts returning memory at the end of the .bss section,
up the alignment of this in the linker script to 16 bytes.

Also enable the 'noabicalls' feature in the target.json. This is the
default in GCC with the mips-elf target, but not for LLVM. The linker
will emit some warnings without this.

Should return the previous pointer, rather than the incremented one.

As described in:
rust-lang/rust#28728