Add comment explaining why libunwind doesn't need to link libgcc_eh

Author: joshtriplett

library/unwind/src/lib.rs

@@ -42,6 +42,9 @@ cfg_if::cfg_if! {
 #[link(name = "gcc_s", cfg(not(target_feature = "crt-static")))]
 extern "C" {}
 
+// When building with crt-static, we get `gcc_eh` from the `libc` crate, since
+// glibc needs it, and needs it listed later on the linker command line. We
+// don't want to duplicate it here.
 #[cfg(all(target_os = "linux", target_env = "gnu", not(feature = "llvm-libunwind")))]
 #[link(name = "gcc_s", cfg(not(target_feature = "crt-static")))]
 extern "C" {}