[scudo] Fix standalone compiler-rt test breakage

[cryptoad]

Summary

Apparently "test standalone compiler-rt" still requires -ldl and -lrt for
Scudo even with --gc-sections. I am not entirely sure why, so if anybody has
some input, feel free to chime in.

In the meantime, add again those two to fix the test.

[kcc]

LGTM

test/scudo/lit.cfg

 # C flags.
 c_flags = ([config.target_cflags] +
            ["-std=c++11",
            "-pthread",
            "-fPIE",
            "-pie",
            "-O0",
            "-UNDEBUG",
+           "-ldl",
+           "-lrt",
            "-Wl,--gc-sections"])
 
 def build_invocation(compile_flags):                                            
   return " " + " ".join([config.clang] + compile_flags) + " "                   
 
 # Add clang substitutions.
 config.substitutions.append( ("%clang_scudo ",
                               build_invocation(c_flags) + whole_archive) )
 
 # Hardened Allocator tests are currently supported on Linux only.
 if config.host_os not in ['Linux']:
    config.unsupported = True