miniruby is used by tool/runruby.rb, so we need to ensure we don't rb_bug
when RUBY_GC_LIBRARY_PATH is set so we can run tests using the make
commands. This commit changes it to warn instead.

On Windows, the error exists on the stack so we should pass an error
buffer from the caller.

Before, if dln_open failed to open RUBY_GC_LIBRARY_PATH, it would segfault
because it would try to raise an error, which cannot happen because the
GC has not been initialized yet.

This commit changes dln_open to return the error that occurred so the
caller can handle the error.

Co-Authored-By: Peter Zhu <peter@peterzhu.ca>

They are used conditionally in miniruby, dynamically-linked ruby, and
statically-linked ruby (--with-static-linked-ext).
I asked @nobu about the conditions. and I leave comments because the
conditions are insanely difficult.

The symbol resolved by dln_symbol will eventually be passed to
extensions. The error handling of dln_sym is also separated into
dln_sym_func because the new call resolving symbols will not raise
LoadError.

* include/ruby/ruby.h (UNREACHABLE_RETURN): UNREACHABLE at the end
  of non-void functions.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64025 b2dd03c8-39d4-4d8f-98ff-823fe69b080e