cmd/link: shrink SymKind down to a uint8

Change-Id: I54c73afd4fb2ad4d5854d2568524f57542c64fe5
Reviewed-on: https://go-review.googlesource.com/42182
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/link/internal/sym/symkind.go

@@ -31,7 +31,7 @@
 package sym
 
 // A SymKind describes the kind of memory represented by a symbol.
-type SymKind int16
+type SymKind uint8
 
 // Defined SymKind values.
 //