[scudo] Use gc-sections by default

[cryptoad]

Summary

If not using -Wl,--gc-sections, a whole lot of unused sanitizer_common code
and related static variables are pulled into the shared library.
Keep the binary size smaller, and its memory footprint smaller as well, by
using the compiler flags -ffunction-section & -fdata-sections by default,
as well as the linker flags -Wl,--gc-sections.

Current experiments show a large discrepency between binary sizes generated
by gcc (big) and clang (small). I am not sure yet how I can make a test that
would encompass both, so it's an outstanding work item.

lib/scudo/CMakeLists.txt

 add_compiler_rt_component(scudo)
 
 include_directories(..)
 
 set(SCUDO_CFLAGS ${SANITIZER_COMMON_CFLAGS})
 # SANITIZER_COMMON_CFLAGS include -fno-builtin, but we actually want builtins!
 list(APPEND SCUDO_CFLAGS -fbuiltin)
 append_rtti_flag(OFF SCUDO_CFLAGS)
 
 set(SCUDO_DYNAMIC_LINK_FLAGS ${SANITIZER_COMMON_LINK_FLAGS})
 
+# Use gc-sections by default to avoid unused code being pulled in.
+list(APPEND SCUDO_CFLAGS -ffunction-sections -fdata-sections)
+list(APPEND SCUDO_DYNAMIC_LINK_FLAGS -Wl,--gc-sections)
+
 set(SCUDO_SOURCES
   scudo_allocator.cpp
   scudo_crc32.cpp
   scudo_flags.cpp
   scudo_malloc.cpp
   scudo_termination.cpp
   scudo_tsd_exclusive.cpp
   scudo_tsd_shared.cpp