make: rustc invocations need to depend on the rustc libraries

[pnkfelix]

(creating issue to match PR #7820, since I worry about that growing stale, and there are some things like labels that you can use with filed Issues that you cannot use with pull requests.)

As written in Makefile.in, $(STAGE$(1)T$(2)H$(3)) is the way one
writes an invocation of rustc where $(1) is the stage number $(2) is
the target triple $(3) is the host triple. (Other uses of the macro
may plug in actual values or different parameters in for those three
formal parameters.)

When you have invocations of $(STAGE...), you need to make sure that
its dependences are satisfied; otherwise, if someone is using make -jN for certain (large-ish) N, one can encounter situations where
GNU make attempts to invoke rustc before it has actually copied some
of its libraries into place, such as libmorestack.a, which causes a
link failure when the rustc invocation attempts to link in those
libraries.

In this case, the main prerequisite to add is TSREQ$(1)T$(2)H$(3),
which is described in Makefile.in as "Prerequisites for using the
stageN compiler to build target artifacts"

[pnkfelix]

Part of #8058