[tests] Add back stdc macros I accidentally refactored out.

- Patch by Michael van der Westhuizen:
--
r174404 accidentally removed stdc format, limit and constant macros from the Linux test runner logic.  This small patch re-adds the macros.

Making this change fixes the following tests on Linux:
 - depr/depr.c.headers/inttypes_h.pass.cpp
 - depr/depr.c.headers/stdint_h.pass.cpp
 - input.output/file.streams/c.files/cinttypes.pass.cpp
 - language.support/cstdint/cstdint.syn/cstdint.pass.cpp
--

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174722 91177308-0d34-0410-b5e6-96231b3b80d8

test/lit.cfg

@@ -276,6 +276,7 @@ elif sys.platform == 'linux2':
     libraries += ['-lsupc++', '-lgcc_eh', '-lc', '-lm', '-lpthread', '-lrt', '-lgcc_s']
     if not use_system_lib:
         libraries += ['-Wl,-R', libcxx_obj_root + '/lib']
+    compile_flags += ['-D__STDC_FORMAT_MACROS', '-D__STDC_LIMIT_MACROS', '-D__STDC_CONSTANT_MACROS']
 else:
     lit.fatal("unrecognized system")