crypto/rand: use getrandom system call on Linux

Adds internal/syscall package.

Fixes Issue 8520

LGTM=r, agl
R=agl, rsc, r
CC=golang-codereviews, iant
https://codereview.appspot.com/123260044
doc: document go1.3.1

LGTM=r
R=r, rsc
CC=golang-codereviews
https://codereview.appspot.com/126060043
cmd/go, go/build: implement import comment checking

See golang.org/s/go14customimport for design.

Added case to deps_test to allow go/build to import regexp.
Not a new dependency, because go/build already imports go/doc
which imports regexp.

Fixes issue 7453.

LGTM=r
R=r, josharian
CC=golang-codereviews
https://codereview.appspot.com/124940043
cmd/ld: handle large link base addresses

codeblk and datblk were truncating their
arguments to int32. Don't do that.

LGTM=dvyukov, rminnich
R=iant, dvyukov, rminnich
CC=golang-codereviews
https://codereview.appspot.com/126050043