cargo: support building as part of libstd

Support using `unwinding` as a backend for the libstd `unwind` crate.
This will enable unwinding support for new targets while allowing us to
continue to use `libunwind` from llvm for supported targets.

Signed-off-by: Sean Cross <sean@xobs.io>

Cargo.toml

@@ -14,6 +14,8 @@ members = ["cdylib", "example"]
 gimli = { version = "0.26.1", default-features = false, features = ["read-core"] }
 libc = { version = "0.2", optional = true }
 spin = { version = "0.9.8", optional = true, default-features = false, features = ["mutex", "spin_mutex"] }
+core = { version = '1.0.0', optional = true, package = 'rustc-std-workspace-core' }
+compiler_builtins = { version = "0.1.2", optional = true }
 
 [features]
 alloc = []
@@ -36,6 +38,7 @@ panic-handler = ["print", "panic"]
 panic-handler-dummy = []
 system-alloc = []
 default = ["unwinder", "dwarf-expr", "hide-trace", "fde-phdr-dl", "fde-registry"]
+rustc-dep-of-std = ["core", "gimli/rustc-dep-of-std", "compiler_builtins"]
 
 [profile.release]
 debug = true