So different timestamps for different paths will be used.  Extentions
paths in bundled gems contain `ruby_version`, which includes the ABI
version, and the same timestamp file for different paths resulted in
build failures when it changed.

Adding `ruby` to `PREP` causes the following circular dependencies
because `PREP` is used as a prerequisite by some targets required to
build `ruby` target itself.
```
make: Circular .rbconfig.time <- ruby dependency dropped.
make: Circular builtin_binary.inc <- ruby dependency dropped.
make: Circular ext/extinit.c <- ruby dependency dropped.
make: Circular ruby <- ruby dependency dropped.
```

Adding a new Make variable like `EXTPREP` only for exts may be also
reasonable, but it would introduce another complexity into our build
system. `-bundle_loader` doesn't care that link-time and run-time
loader executables are different as long as bound symbols are provided,
so it's ok to resolve from miniruby to simplify our build.

For the macOS -bundle_loader linker option, we need a path to the
Ruby exectuable. $(RUBY) is not necessarily a path since it could
be a command line invocation. That happens during build with
runruby.rb and can happen post installation if the user passes
the --ruby option to a extconf.rb. Use $(bindir) to locate
the executable instead.

Before installation, $(bindir) doesn't exist, so we need to be
able to override $(BUILTRUBY) in such situations so test-spec
and bundled extensions could build. Use a new mkmf global,
$builtruby, to do this; set it in fake.rb and in extmk.rb.

Our build system is quite complex...

ld64 shipped with Xcode 14 emits a warning when using `-undefined
dynamic_lookup`.

```
ld: warning: -undefined dynamic_lookup may not work with chained fixups
```

Actually, `-undefined dynamic_lookup` doesn't work when:

1. Link a *shared library* with the option
2. Link it with a program that uses the chained-fixup introduced from
   macOS 12 and iOS 15
because `-undefined dynamic_lookup` uses lazy-bindings and they won't be
bound while dyld fixes-up by traversing chained-fixup info.

However, we build exts as *bundles* and they are loaded only through
`dlopen`, so it's safe to use `-undefined dynamic_lookup` in theory.
So the warning produced by ld64 is false-positive, and it results
failure of option checking in configuration. Therefore, it would be an
option to ignore the warning during our configuration.

On the other hand, `-undefined dynamic_lookup` is already deprecated on
all darwin platforms except for macOS, so it's good time to get rid of
the option. ld64 also provides `-bundle_loader <executable>` option,
which allows to resolve symbols defined in the executable symtab while
linking. It behaves almost the same with `-undefined dynamic_lookup`,
but it makes the following changes:

1. Require that unresolved symbols among input objects must be defined
   in the executable.
2. Lazy symbol binding will lookup only the symtab of the bundle loader
   executable. (`-undefined dynamic_lookup` lookups all symtab as flat
   namespace)

This patch adds `-bundle_loader $(RUBY)` when non-EXTSTATIC
configuration by assuming ruby executable can be linked before building
exts.

See "New Features" subsection under "Linking" section for chained fixup
https://developer.apple.com/documentation/xcode-release-notes/xcode-13-release-notes