cmd/internal/gc: convert Val.U to interface{}

cmd/internal/gc: convert Val.U to interface{}

This CL was generated by updating Val in go.go
and then running:

sed -i "" 's/\.U\.[SBXFC]val = /.U = /' *.go
sed -i "" 's/\.U\.Sval/.U.\(string\)/g' *.go *.y
sed -i "" 's/\.U\.Bval/.U.\(bool\)/g' *.go *.y
sed -i "" 's/\.U\.Xval/.U.\(\*Mpint\)/g' *.go *.y
sed -i "" 's/\.U\.Fval/.U.\(\*Mpflt\)/g' *.go *.y
sed -i "" 's/\.U\.Cval/.U.\(\*Mpcplx\)/g' *.go *.y

No functional changes. Passes toolstash -cmp.

This reduces the size of gc.Node from 424 to 392 bytes.
This in turn reduces the permanent (pprof -inuse_space)
memory usage while compiling the test/rotate?.go tests:

test	old(MB)	new(MB)	change
rotate0	379.49	364.78	-3.87%
rotate1	373.42	359.07	-3.84%
rotate2	381.17	366.24	-3.91%
rotate3	374.30	359.95	-3.83%

CL 8445 was similar to this; gri asked that Val's implementation
be hidden first. CLs 8912, 9263, and 9267 have at least
isolated the changes to the cmd/internal/gc package.

Updates #9933.

Change-Id: I83ddfe003d48e0a73c92e819edd3b5e620023084
Reviewed-on: https://go-review.googlesource.com/10059
Reviewed-by: Russ Cox <rsc@golang.org>