Add .arcconfig file. Do not use yet.

Don't use this yet - it's an experiment, a disaster, whatever. Caveat
emptor. I'll probably delete everything. Do not taunt Happy Fun Ball -
you have been warned.

Signed-off-by: Austin Seipp <austin@well-typed.com>
Add ".text.unlikely" to recognized code sections on Windows.

Fixes #9080

Signed-off-by: Austin Seipp <austin@well-typed.com>
Emit error in case of duplicate GRE; fixes #7241

Signed-off-by: Austin Seipp <austin@well-typed.com>