[TOOLS] Fix foreign header build breakage.

[TOOLS] Fix foreign header build breakage.

Having both the Xen tree and the tools tree recurse into the foreign
headers directory causes headaches in parallel builds and when
building 32-bit tools + 64 bit Xen in the same tree.

Therefore we cause mk-symlinks to symlink in the foreign headers build
bits and generate a local version of the headers and checker tool.

Only libxc needs to do this since the other tools were actually
picking up the libxc version of the headers anyway so the mk-symlinks
calls can be removed from these components.

Signed-off-by: Ian Campbell <ian.campbell@xensource.com>