runtime: top align tinyallocs in race mode

Top align allocations in tinyalloc buckets when in race mode.
This will make checkptr checks more reliable, because any code
that modifies a pointer past the end of the object will trigger
a checkptr error.

No test, because we need -race for this to actually kick in.  We could
add it to the race detector tests, but the race detector tests are all
geared towards race detector reports, not checkptr reports. Mucking
with parsing reports is more than a test is worth.

Fixes #38872

Change-Id: Ie56f0fbd1a9385539f6631fd1ac40c3de5600154
Reviewed-on: https://go-review.googlesource.com/c/go/+/315029


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>