Make panic-unwind a default feature for libstd

x.py sets it unconditionally, so want it for plain "cargo build".

We need to load one of the panic runtimes that is in src (vs. pre-built in the
compiler's sysroot) to ensure that we don't load libpanic_unwind from the
sysroot. That would lead to a load of libcore, also from the sysroot, and create
lots of errors about duplicate lang items.

src/libstd/Cargo.toml

@@ -47,7 +47,7 @@ hermit-abi = { version = "0.1.10", features = ['rustc-dep-of-std'] }
 wasi = { version = "0.9.0", features = ['rustc-dep-of-std'], default-features = false }
 
 [features]
-default = ["std_detect_file_io", "std_detect_dlsym_getauxval"]
+default = ["std_detect_file_io", "std_detect_dlsym_getauxval", "panic-unwind"]
 
 backtrace = [
   "backtrace_rs/dbghelp",          # backtrace/symbolize on MSVC