go/types, cmd/compile: skip crypto/internal/bigmod/_asm in TestStdlib

go/types, cmd/compile: skip crypto/internal/bigmod/_asm in TestStdlib

Similar to CL 317869, apply to a newly added directory.

Maybe we should skip all directories starting with "_"?

Updates #46027.

Change-Id: Idcb011fda877c9f2cb3032524bebfcc0c1da70b2
Reviewed-on: https://go-review.googlesource.com/c/go/+/452437
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>