build: support target sysroot

This allows two things:

  - We can do a cross-compile with clang 😁
  - We can do a cross-compiler with gcc against a different sysroot

This also fixes up the cross-compilation for gcc as it correctly sets
the `target_*` variables in the target block.

Change-Id: Ic1ff2d5732dc6ef3aded24c87669c7620ab4f0b2