build: fix cross-compile without CPU emulation

Fix cross compiling where CPU emulation is not needed, for example building x86 packages on x86_64 machines.

This is the case with both:

• CrossCompile.UNNECESSARY
• arch != Arch.native()

Fix the logic in CrossCompile().build_chroot that returned the native chroot in that case instead of Chroot.buildroot(arch).

Fix error:

/usr/bin/abuild: line 2692: x86_64-alpine-linux-musl-strip: not found

Fix build.postmarketos.org#155 (closed)