asmcheck comments now support a compact form of specifying
multiple checks for each platform, using the following syntax:

   amd64:"SHL\t[$]4","SHR\t[$]4"

Negative checks are also parsed using the following syntax:

   amd64:-"ROR"

though they are still not working.

Moreover, out-of-line comments have been implemented. This
allows to specify asmchecks on comment-only lines, that will
be matched on the first subsequent non-comment non-empty line.

    // amd64:"XOR"
    // arm:"EOR"

    x ^= 1

Change-Id: I110c7462fc6a5c70fd4af0d42f516016ae7f2760
Reviewed-on: https://go-review.googlesource.com/97816Reviewed-by: Keith Randall <khr@golang.org>