symbolz: skip un-symbolizable mappings

This fixes google#339 by skipping
attempts to symbolize anything from system mappings like [vdso],
[vsyscall], etc.

internal/symbolz/symbolz.go

@@ -38,13 +38,18 @@ var (
 // symbolz handler. syms receives the symbolz query (hex addresses
 // separated by '+') and returns the symbolz output in a string. If
 // force is false, it will only symbolize locations from mappings
-// not already marked as HasFunctions.
+// not already marked as HasFunctions. Never attempts symbolization of
+// any symbols found in unsymbolizable system mappings.
 func Symbolize(p *profile.Profile, force bool, sources plugin.MappingSources, syms func(string, string) ([]byte, error), ui plugin.UI) error {
 	for _, m := range p.Mapping {
 		if !force && m.HasFunctions {
 			// Only check for HasFunctions as symbolz only populates function names.
 			continue
 		}
+		// Skip well-known system mappings.
+		if m.Unsymbolizable() {
+			continue
+		}
 		mappingSources := sources[m.File]
 		if m.BuildID != "" {
 			mappingSources = append(mappingSources, sources[m.BuildID]...)