compile backtrace-related deps with opt-level=s

Locally, this shrinks a stripped helloworld by 8k (~2%),
which isn't huge but it's something.

I also tried `opt-level=z` but it produces the same result.
`opt-level=s` is preferrable in that case since it inhibits fewer
optimizations.

Cargo.toml

@@ -105,6 +105,16 @@ adler.debug = 0
 gimli.debug = 0
 miniz_oxide.debug = 0
 object.debug = 0
+rustc-demangle.debug = 0
+# Similar to above, as these libraries are used for backtrace symbolication,
+# we can afford to sacrifice a small amount of performance in order to get
+# smaller binaries by optimizing for size.
+addr2line.opt-level = 's'
+adler.opt-level = 's'
+gimli.opt-level = 's'
+miniz_oxide.opt-level = 's'
+object.opt-level = 's'
+rustc-demangle.opt-level = 's'
 
 # These are very thin wrappers around executing lld with the right binary name.
 # Basically nothing within them can go wrong without having been explicitly logged anyway.