runtime: eliminate some unnecessary uintptr conversions

runtime: eliminate some unnecessary uintptr conversions

arena_{start,used,end} are already uintptr, so no need to convert them
to uintptr, much less to convert them to unsafe.Pointer and then to
uintptr.  No binary change to pkg/linux_amd64/runtime.a.

Change-Id: Ia4232ed2a724c44fde7eba403c5fe8e6dccaa879
Reviewed-on: https://go-review.googlesource.com/16339
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>