[dev.link] cmd/link/internal/loader: reduce ABI alias postprocessing …

…overhead

In loadObjFull when populating the sym.Reloc vector for live symbols,
avoid using the loader.SymType() method to determine if a relocation
targets an ABI alias; since invoking loader.SymType requires a
global-to-local index translation and a read from the object file.
Instead just look at the target symbol itself, which has already been
created at this point.

Hyperkube performance numbers for this change:

name                         old time/op  new time/op  delta
RelinkHyperkube               29.9s ± 2%   29.2s ± 3%  -2.42%  (p=0.000 n=20+20)
RelinkWithoutDebugHyperkube   22.0s ± 3%   21.4s ± 3%  -2.58%  (p=0.000 n=20+20)

Change-Id: Ib7696d8760dd0485240246d6d640668fbf451d71
Reviewed-on: https://go-review.googlesource.com/c/go/+/205257
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/cmd/link/internal/loader/loader.go

@@ -1058,7 +1058,7 @@ func loadObjFull(l *Loader, r *oReader) {
 				rs = 0
 				sz = 0
 			}
-			if rs != 0 && l.SymType(rs) == sym.SABIALIAS {
+			if rs != 0 && l.Syms[rs] != nil && l.Syms[rs].Type == sym.SABIALIAS {
 				rsrelocs := l.Relocs(rs)
 				rs = rsrelocs.At(0).Sym
 			}