build: pass over a host that has no mrbc to lend - #7240

A cross target borrows `mrbc` from a build that answers `MRB_NO_FLOAT` the way
it does, because `src/load.c` refuses a whole irep over a pool entry the
target cannot represent. The question was asked of the compiler alone, and a
build config can write the define on the build:

```ruby
MRuby::CrossBuild.new('target') do |conf|
  conf.defines << 'MRB_NO_FLOAT'
end
```

`Command::Compiler#all_flags` puts `build.defines` on every command line, so
the target compiles without floats while `cc.has_define?` says it does not. It
borrowed a `host` that answers otherwise, and the `mrbc` of that host wrote a
float pool entry the target refused to load:

```console
$ rake -m test
.........(unknown):0: irep load error (ScriptError)
```

Read both lists, the way `Build#has_define?` reads them. `Build#has_define?`
itself cannot be asked here: it refuses until the gems are set up, and a cross
build binds its `mrbc` as it is declared.

The question also moves to `MRBC_DEFINES`, which the comparison and the
defines the generated build carries now both read, so a second define that
decides what a pool entry may hold is one entry rather than three places.

A cross build borrows `mrbc` from a declared `host` on one question:
whether it answers `MRB_NO_FLOAT` the way the target does. It never asks
whether that `host` has a `mrbc` at all, so a `host` with
`disable_libmruby` and no `mruby-bin-mrbc` is borrowed anyway, and the
target's own `mrblib` rule fails inside `Build#mrbcfile` with a message
that names `host` twice and the target not at all:

```ruby
MRuby::Build.new('host') do |conf|
  conf.toolchain
  conf.disable_libmruby
end

MRuby::CrossBuild.new('cross') do |conf|
  conf.toolchain
  conf.gem :core => "mruby-bin-mruby"
end
```

```console
external mrbc or mruby-bin-mrbc gem in current('host') or 'host' build is required
```

Everywhere else `bind_mrbc_host` generates a build when the declared
`host` will not do: where there is none, and where it answers otherwise
on the defines in `MRBC_DEFINES`. Only "it has none to lend" fell
through to the raise.

Ask that too. `Build#supplies_mrbc?` says whether a build has a `mrbc`
to hand out, which is the question `Build#mrbcfile` already puts to
`host` on behalf of a native build: one it was given, one
`create_mrbc_build` generated for it, or one it builds from the gem.
`bind_mrbc_host` asks it before the defines are compared, and a `host`
that answers no is passed over for the generated build the answer
names, `mrbc/default` here, the way a `host` that answers otherwise on
the defines already is. The `host` itself is left as it is written.

The config above now builds, with `mrbc` generated under `build/mrbc`.
A `host` that carries the gem, one given an external `mrbcfile`, one
with `libmruby`, and a config with no `host` bind as before, and a cross
build with its own external `mrbcfile` still asks nothing of `host`.