Pass user-specified options to ghc-toolchain

The current user interface to configuring target toolchains is `./configure`.

In !9263 we added a new tool to configure target toolchains called
`ghc-toolchain`, but the blessed way of creating these toolchains is
still through configure.

However, we were not passing the user-specified options given with the
`./configure` invocation to the ghc-toolchain tool.

This commit remedies that by storing the user options and environment
variables in USER_* variables, which then get passed to GHC-toolchain.

The exception to the rule is the windows bundled toolchain, which
overrides the USER_* variables with whatever flags the windows bundled
toolchain requires to work.
We consider the bundled toolchain to be effectively the user specifying
options, since the actual user delegated that configuration work.

Closes #23678