[compiler-rt] Make use of undefined symbols configurable

[phosek]

Summary

We want to disable the use of undefined symbols on Fuchsia, but there
are cases where it might be desirable so may it configurable.

[mcgrathr]

Since those only affects sanitizers and not other compiler-rt libraries, perhaps it should be called SANITIZER_* instead of COMPILER_RT_*?

[phosek]

In D104728#2833958, @mcgrathr wrote:

Since those only affects sanitizers and not other compiler-rt libraries, perhaps it should be called SANITIZER_* instead of COMPILER_RT_*?

Done

compiler-rt/CMakeLists.txt

else()
   if (ANDROID)
     append_list_if(COMPILER_RT_HAS_GCC_LIB gcc SANITIZER_COMMON_LINK_LIBS)
   else()
     append_list_if(COMPILER_RT_HAS_GCC_S_LIB gcc_s SANITIZER_COMMON_LINK_LIBS)
   endif()
 endif()
 
 append_list_if(COMPILER_RT_HAS_LIBC c SANITIZER_COMMON_LINK_LIBS)
+if("${CMAKE_SYSTEM_NAME}" STREQUAL "Fuchsia")
+  list(APPEND SANITIZER_COMMON_LINK_LIBS zircon)
+endif()
 
 if("${CMAKE_SYSTEM_NAME}" STREQUAL "Fuchsia")
+  set(SANITIZER_NO_UNDEFINED_SYMBOLS_DEFAULT ON)
+else()
+  set(SANITIZER_NO_UNDEFINED_SYMBOLS_DEFAULT OFF)
+endif
+option(SANITIZER_NO_UNDEFINED_SYMBOLS ${SANITIZER_NO_UNDEFINED_SYMBOLS_DEFAULT})
+if (SANITIZER_NO_UNDEFINED_SYMBOLS)
   list(APPEND SANITIZER_COMMON_LINK_FLAGS -Wl,-z,defs)
-  list(APPEND SANITIZER_COMMON_LINK_LIBS zircon)
 endif()
 
 # TODO: COMPILER_RT_COMMON_CFLAGS and COMPILER_RT_COMMON_LINK_FLAGS are
 # intended for use in non-sanitizer runtimes such as libFuzzer, profile or XRay,
 # move these higher to include common flags, then derive SANITIZER_COMMON_CFLAGS
 # and SANITIZER_COMMON_LINK_FLAGS from those and append sanitizer-specific flags.
 set(COMPILER_RT_COMMON_CFLAGS ${SANITIZER_COMMON_CFLAGS})
 set(COMPILER_RT_COMMON_LINK_FLAGS ${SANITIZER_COMMON_LINK_FLAGS})