Fix getall failure with MMTk dependency (#57038)
This PR fixes https://github.com/JuliaLang/julia/issues/57002. The
current makefile checks if `MMTK_JULIA_DIR` points to
`$(BUILDROOT)/usr/lib/mmtk_julia` and defines the rule `get-mmtk_julia`
if the condition is met. For non-mmtk builds (e.g. `make -C deps
getall`), `MMTK_JULIA_DIR` is not defined, thus `get-mmtk_julia` is not
defined. This PR moves the rule `get-mmtk_julia` outside the condition
so it always exists.
