This is a step toward using cached build artifacts: the importcfg
will direct the compiler and linker to read them right from the cache
if necessary. However, this CL does not have a cache yet, so it still
reads them from the usual install location or build location.
Even so, this fixes a long-standing issue that -I and -L (no longer used)
are not expressive enough to describe complex GOPATH setups.

Shared libraries are handled enough that all.bash passes, but
there may still be more work to do here. If so, tests and fixes
can be added in follow-up CLs.

Gccgo will need updating to support -importcfg as well.

Fixes #14271.

Change-Id: I5c52a0a5df0ffbf7436e1130c74e9e24fceff80f
Reviewed-on: https://go-review.googlesource.com/56279
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>