Skip these tests at stage1.

Note: apparently you cannot quote operands to `ifeq` in Makefile.
Factored out commands to delete dylib/rlib into REMOVE_DYLIBS/REMOVE_RLIBS.

There were places where we were only calling `rm $(call DYLIB,foo)`
even though we really needed to get rid of the whole glob (at least
based on alex's findings on #13753 that removing the symlink does not
suffice).

Therefore rather than peppering the code with the awkward `rm
$(TMPDIR)/$(call DYLIB_GLOB,foo)`, I instead introduced a common
`REMOVE_DYLIBS` user function that expands into that when called.
After I adding an analogous `REMOVE_RLIBS`, I changed all of the
existing calls that rm dylibs or rlibs to use these routines instead.

Note that the latter is not a true refactoring since I may have
changed cases where it was our intent to only remove the sym-link.
(But if that is the case, then we need to more deeply investigate
alex's findings on #13753 where the system was still dynamically
loading up the non-symlinked libraries that it finds on the load
path.)
Fixing rustdoc stage1.

See #13983 and #14000.

Conflicts:
	src/librustc/metadata/filesearch.rs
	src/libstd/unstable/dynamic_lib.rs
Excuse regex from doing rustdoc tests atop stage1.

Hopefully final baby-step to getting `make check-stage1` working
again.

The use of a special-case check for regex here is ugly but is
analogous other similar checks for regex such as the one that landed
in PR #13844.

The way this is written, the user will get a reminder that
doc-crate-regex is being skipped whenever their rules attempt to do
the crate documentation tests.  This is deliberate: I want people
running `make check-stage1` to be reminded about which cases are being
skipped.  (But if such echo noise is considered offensive, it can
obviously be removed.)