hadrian: Generate HSC2HS_EXTRAS variable in bindist installation

We must generate the hsc2hs wrapper at bindist installation time since
it must contain `--lflag` and `--cflag` arguments which depend upon the
installation path.

The solution here is to substitute these variables in the configure
script (see mk/hsc2hs.in). This is then copied over a dummy wrapper in
the install rules.

Fixes #24050.