build.rs: aarch64: Specify target for cc::Build

Oddly, not specifying this or using aarch64-unknown-none (which would be
the default that cc gets from the TARGET environment variable) both
fail to invoke the appropriate compiler to build the asm code.

Using aarch64-unknown-redox works but shouldn't really be needed. This
is perhaps because of some odd arrangement of KTARGET, TARGET, the
installed prefix toolchain and the kernel target JSON spec.

The early_init asm code shall be replaced by a pure Rust bootloader
eventually so let's move with this for the moment.