Enable folly::Symbolizer for debug symbols

This change depends on hhvm/hhvm-third-party#123,
which enables 2 additional source files to be compiled and adds libunwind as a
dependency for folly. The "-lgcc_s" added before "-lunwind" is used to solve a
linking order issue that causes "_Ux86_64_setcontext" to be called from
libunwind instead of libgcc_s and messes up the signal mask
(more details here http://savannah.nongnu.org/bugs/?48486). The libgcc_s
dependency is already part of HHVM, this change enforces the order in which
the libraries are linked.

By enabling folly::Symbolizer, HHVM will show debug symbols (in stacktraces,
perf, GDB) when huge pages are enabled (default behavior for open
source builds). Also, one nice side-effect is that this change causes HHVM
to show symbols in backtraces (after a crash) for non-huge-pages builds,
which were missing for some reason.

CMake/Options.cmake

@@ -31,7 +31,10 @@ option(ENABLE_PROXYGEN_SERVER "Build the Proxygen HTTP server" ON)
 option(ENABLE_SPLIT_DWARF "Reduce linker memory usage by putting debugging information into .dwo files" OFF)
 
 IF (LINUX)
-    option(MAP_TEXT_HUGE_PAGES "Remap hot static code onto huge pages" ON)
+  option(MAP_TEXT_HUGE_PAGES "Remap hot static code onto huge pages" ON)
+  IF(${CMAKE_CXX_COMPILER_ID} STREQUAL "GNU")
+    option(ENABLE_FOLLY_SYMBOLIZER "Use folly::Symbolizer to obtain debug symbols when mapping text section onto huge pages" ON)
+  ENDIF()
 ENDIF()
 
 IF (NOT DEFAULT_CONFIG_DIR)

hphp/CMakeLists.txt

@@ -108,6 +108,10 @@ install(PROGRAMS ${CMAKE_CURRENT_SOURCE_DIR}/tools/oss-repo-mode
 
 add_definitions("-DUSE_CMAKE")
 
+if (ENABLE_FOLLY_SYMBOLIZER)
+    add_definitions(-DUSE_FOLLY_SYMBOLIZER)
+endif()
+
 enable_language(ASM)
 
 link_libraries(folly)

hphp/util/portability.h

@@ -231,7 +231,6 @@
 //////////////////////////////////////////////////////////////////////
 
 #if FACEBOOK
-#define USE_FOLLY_SYMBOLIZER 1
 // Linking in libbfd is a gigantic PITA, but if folly symbolizer doesn't
 // work on your platform, you'll need to figure it out.
 #define HAVE_LIBBFD 1