runtime: rename gcpercent, readgogc, and heapminimum to match Go style

Generated with:

rf 'mv gcpercent gcPercent'
rf 'mv readgogc readGOGC'
rf 'mv heapminimum heapMinimum'

After this, comments referencing these symbols were updated via a simple
sed command.

For #44167.

Change-Id: I6bb01597c2130686c01f967d0f106b06860ad2db
Reviewed-on: https://go-review.googlesource.com/c/go/+/306597


Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>