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>