runtime: align m.procid to 8 bytes on 32-bit systems

https://go-review.googlesource.com/c/go/+/383434 started using
atomic Load64 on this field, which breaks 32 bit platforms which
require 64-bit alignment of uint64s that are passed to atomic operations.

Not sure why this doesn't break everywhere, but I saw it break on
my laptop during all.bash.

Change-Id: Ida27b23068b3cc7208fce3c97b69a464ccf68209
Reviewed-on: https://go-review.googlesource.com/c/go/+/399754
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
(cherry picked from commit ea7e3e3)

src/runtime/runtime2.go

@@ -514,6 +514,7 @@ type m struct {
 	g0      *g     // goroutine with scheduling stack
 	morebuf gobuf  // gobuf arg to morestack
 	divmod  uint32 // div/mod denominator for arm - known to liblink
+	_       uint32 // align next field to 8 bytes
 
 	// Fields not known to debuggers.
 	procid        uint64            // for debuggers, but offset not hard-coded