This commit adds the host_fully_static flavour transformer to hadrian,
which ensures stage0 is fully statically linked while still permitting
stage1 libdir to contain shared libraries. This is intended to be used
by the wasm backend to build portable linux bindists that contain wasm
shared libraries.

(cherry picked from commit b45080a3)
(cherry picked from commit 91c0dbd4)
(cherry picked from commit e077d581)