Fix compiler-rt build on musl

[raj.khem]

Summary

stddef.h is needed for size_t definition. this is exposed when building compiler-rt for musl C library on Linux, this is also needed for clang-12 release branch

[tstellar]

Can you file a bug for this and mark it as a blocker for release-12.0.0?

[MaskRay]

compiler-rt/lib/fuzzer/FuzzerInterceptors.cpp

29

Please move it before cstdint

[raj.khem]

compiler-rt/lib/fuzzer/FuzzerInterceptors.cpp

29

done

[raj.khem]

@MaskRay I dont find this in main branch yet, anything pending on me ?

compiler-rt/lib/fuzzer/FuzzerInterceptors.cpp

 #define FUNC_TYPE(x) x##_type
 #define DEFINE_REAL(ret_type, func, ...)                                       \
   typedef ret_type (*FUNC_TYPE(func))(__VA_ARGS__);                            \
   namespace __interception {                                                   \
   FUNC_TYPE(func) PTR_TO_REAL(func);                                           \
   }
 
 #include <cassert>
+#include <cstddef> // for size_t
 #include <cstdint>
 #include <dlfcn.h> // for dlsym()
 
 static void *getFuncAddr(const char *name, uintptr_t wrapper_addr) {
   void *addr = dlsym(RTLD_NEXT, name);
   if (!addr) {
     // If the lookup using RTLD_NEXT failed, the sanitizer runtime library is
     // later in the library search order than the DSO that we are trying to
     // intercept, which means that we cannot intercept this function. We still