Make `_Unwind_Action` a type alias, not enum
It's bitflags in practice, so an enum is unsound, as an enum must only have the described values. The x86_64 psABI declares it as a `typedef int _Unwind_Action`, which seems reasonable. I made a newtype first but that was more annoying than just a typedef. We don't really use this value for much other than a short check.
I ran `x check library --target aarch64-unknown-linux-gnu,x86_64-pc-windows-gnu,x86_64-fortanix-unknown-sgx,x86_64-unknown-haiku,x86_64-unknown-fuchsi
a,x86_64-unknown-freebsd,x86_64-unknown-dragonfly,x86_64-unknown-netbsd,x86_64-unknown-openbsd,x86_64-unknown-redox,riscv64-linux-android,armv7-unknown-freebsd` (and some more but they failed to build for other reasons :D)
fixes#138558
r? workingjubilee have fun
5144055055