Enable to build target without mruby-require

I got bad range error at line 52 in mrbgem.rake when I tried to cross compile mruby-require.
Objective target needs MR, but host target don't.

This task is premised on the existence of MR,
but some of all targets may not use mruby-require.

I can skip all tasks if MR is not found,
but I'm afraid for some capital processing may be added after skipping in the future.