Enable -Zfunction-sections by default for *-windows-{gnu,cygwin}

The comment here was a bit misleading, #13846 was about an LLVM assertion
that failed when `-Zfunction-sections` was enabled. However, that bug
has been fixed for a long time and the actual issue was that older `ld`
versions would not correctly link binaries (see #75604 for the prior
attempt at this change). This bug was fixed in version 2.32 of binutils
and Rust now ships binutils 2.42 as of #119229 so we can remove this
workaround!

Author: wesleywiser

compiler/rustc_target/src/spec/base/cygwin.rs

@@ -26,8 +26,6 @@ pub(crate) fn opts() -> TargetOptions {
     TargetOptions {
         os: "cygwin".into(),
         vendor: "pc".into(),
-        // FIXME(#13846) this should be enabled for cygwin
-        function_sections: false,
         linker: Some("gcc".into()),
         dynamic_linking: true,
         dll_prefix: "".into(),

compiler/rustc_target/src/spec/base/windows_gnu.rs

@@ -80,8 +80,6 @@ pub(crate) fn opts() -> TargetOptions {
         os: "windows".into(),
         env: "gnu".into(),
         vendor: "pc".into(),
-        // FIXME(#13846) this should be enabled for windows
-        function_sections: false,
         linker: Some("gcc".into()),
         dynamic_linking: true,
         dll_tls_export: false,