Add Mac 11.1 SDK to the allowlist

Historically we've only had to add one major release a year to the Mac
SDK allowlist, but this year the versioning scheme changed and 11.1 is a
minor update to 11 (instead of 10.x.1 like in years past). Unfortunately
the Xcode tools don't let you just specify '11' like they did with
'10.x', so this is more complicated to fix in a scalable way.

Test: build with xcode 12.3 and macOS SDK 11.1
Change-Id: I2e88e9289727261389a863e7147a0b146ec5148a

cc/config/x86_darwin_host.go

@@ -67,6 +67,7 @@ var (
 		"10.14",
 		"10.15",
 		"11.0",
+		"11.1",
 	}
 
 	darwinAvailableLibraries = append(