revert obfuscation of the syscall package

In the hopes that it fixes the sporadic gogarble.txtar failures on Mac,
where rebuilding runtime sometimes leads to a different binary.

For #609.

shared.go

@@ -286,6 +286,10 @@ func appendListedPackages(packages []string, withDeps bool) error {
 //
 // TODO: investigate and resolve each one of these
 var cannotObfuscate = map[string]bool{
+	// See https://github.com/burrowers/garble/issues/609.
+	// Also, how do we write a good regression test?
+	"syscall": true,
+
 	// "unknown pc" crashes on windows in the cgo test otherwise
 	"runtime/cgo": true,
 

testdata/script/gogarble.txtar

@@ -38,7 +38,8 @@ garble build std
 # support ImportMap when linking.
 # Also ensure we are obfuscating low-level std packages.
 garble build -o=out ./stdimporter
-! binsubstr out 'http.ListenAndServe' 'debug.WriteHeapDump' 'time.Now' 'syscall.Listen'
+# ! binsubstr out 'http.ListenAndServe' 'debug.WriteHeapDump' 'time.Now' 'syscall.Listen'
+! binsubstr out 'http.ListenAndServe' 'debug.WriteHeapDump' 'time.Now'
 
 # The same low-level std packages appear in plain sight in regular builds.
 go build -o=out_regular ./stdimporter