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.
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