[release-branch.go1.9] cmd/compile: limit the number of simultaneously opened files...

[release-branch.go1.9] cmd/compile: limit the number of simultaneously opened files to avoid EMFILE/ENFILE errors

If the Go packages with enough source files,it will cause EMFILE/ENFILE error,
Fix this by limiting the number of simultaneously opened files.

Fixes #21621

Change-Id: I8555d79242d2f90771e37e073b7540fc7194a64a
Reviewed-on: https://go-review.googlesource.com/57751
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-on: https://go-review.googlesource.com/63752
Run-TryBot: Russ Cox <rsc@golang.org>