runtime: make _MaxMem an untyped constant

Currently _MaxMem is a uintptr, which is going to complicate some
further changes. Make it untyped so we'll be able to do untyped math
on it before truncating it to a uintptr.

The runtime assembly is identical before and after this change on
{linux,windows}/{amd64,386}.

Updates #18651.

Change-Id: I0f64511faa9e0aa25179a556ab9f185ebf8c9cf8
Reviewed-on: https://go-review.googlesource.com/35251
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/runtime/malloc.go

@@ -148,7 +148,11 @@ const (
 	_MHeapMap_TotalBits = (_64bit*sys.GoosWindows)*35 + (_64bit*(1-sys.GoosWindows)*(1-sys.GoosDarwin*sys.GoarchArm64))*39 + sys.GoosDarwin*sys.GoarchArm64*31 + (1-_64bit)*(32-(sys.GoarchMips+sys.GoarchMipsle))
 	_MHeapMap_Bits      = _MHeapMap_TotalBits - _PageShift
 
-	_MaxMem = uintptr(1<<_MHeapMap_TotalBits - 1)
+	// _MaxMem is the maximum heap arena size minus 1.
+	//
+	// On 32-bit, this is also the maximum heap pointer value,
+	// since the arena starts at address 0.
+	_MaxMem = 1<<_MHeapMap_TotalBits - 1
 
 	// Max number of threads to run garbage collection.
 	// 2, 3, and 4 are all plausible maximums depending