Use strcmp() to compare f_lib and JL_LIBJULIA_DL_LIBNAME (#39005)

This future-proofs us from someone trying lookup a symbol from
`libjulia.dll`, even though that should be quite rare, it may happen.

(cherry picked from commit 0af737c)

src/runtime_ccall.cpp

@@ -30,16 +30,16 @@ extern "C"
 void *jl_get_library_(const char *f_lib, int throw_err) JL_NOTSAFEPOINT
 {
  void *hnd;
+ if (f_lib == NULL)
+ return jl_RTLD_DEFAULT_handle;
 #ifdef _OS_WINDOWS_
  if (f_lib == JL_EXE_LIBNAME)
  return jl_exe_handle;
  if (f_lib == JL_LIBJULIA_INTERNAL_DL_LIBNAME)
  return jl_libjulia_internal_handle;
- if (f_lib == JL_LIBJULIA_DL_LIBNAME)
+ if (strcmp(f_lib, JL_LIBJULIA_DL_LIBNAME) == 0)
  return jl_libjulia_handle;
 #endif
- if (f_lib == NULL)
- return jl_RTLD_DEFAULT_handle;
  JL_LOCK_NOGC(&libmap_lock);
  // This is the only operation we do on the map, which doesn't invalidate
  // any references or iterators.