Also move some related files and rules used for `make install`.

Also move the js directory up in the top-level directory, so that the
linking order for the js library stays the same (fdlibm and ffi going
after the js engine objects)

This has the side effect of moving js/src/target very early in the
compilation tier.

--HG--
rename : js/src/js-config.in => js/src/build/js-config.in
rename : js/src/js.pc.in => js/src/build/js.pc.in
rename : js/src/symverscript.in => js/src/build/symverscript.in
extra : rebase_source : 7f188ee11469c7d352d4d9c90c8ba72c97224652