This CL adds a new internal math package for use by the runtime.
The new package exports a MulUintptr function with uintptr arguments
a and b and returns uintptr(a*b) and whether the full-width product
x*y does overflow the uintptr value range (uintptr(x*y) != x*y).

Uses of MulUinptr in the runtime and intrinsics for performance
will be added in followup CLs.

Updates #21588

Change-Id: Ia5a02eeabc955249118e4edf68c67d9fc0858058
Reviewed-on: https://go-review.googlesource.com/c/91755
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>