[AVR] Merge the 'freestanding' base target spec into AVR base target spec

The 'freestanding' module was only ever used for AVR. It was an
unnecessary layer of abstraction. This commit merges the
'freestanding_base' module into 'avr_gnu_base'.

Author: dylanmckay

src/librustc_target/spec/avr_gnu_base.rs

@@ -18,16 +18,34 @@ pub fn target(target_cpu: String) -> TargetResult {
         options: TargetOptions {
             cpu: target_cpu.clone(),
             exe_suffix: ".elf".to_string(),
+
             linker: Some("avr-gcc".to_owned()),
-            pre_link_args: vec![(LinkerFlavor::Gcc,
-                vec![format!("-mmcu={}", target_cpu)],
+            dynamic_linking: false,
+            executables: true,
+            linker_is_gnu: true,
+            has_rpath: false,
+            position_independent_executables: false,
+            eh_frame_header: false,
+            pre_link_args: vec![(
+                LinkerFlavor::Gcc,
+                vec![
+                    format!("-mmcu={}", target_cpu),
+                    // We want to be able to strip as much executable code as possible
+                    // from the linker command line, and this flag indicates to the
+                    // linker that it can avoid linking in dynamic libraries that don't
+                    // actually satisfy any symbols up to that point (as with many other
+                    // resolutions the linker does). This option only applies to all
+                    // following libraries so we're sure to pass it as one of the first
+                    // arguments.
+                    "-Wl,--as-needed".to_string(),
+                ],
             )]
             .into_iter()
             .collect(),
             late_link_args: vec![(LinkerFlavor::Gcc, vec!["-lgcc".to_owned()])]
                 .into_iter()
                 .collect(),
-            ..super::freestanding_base::opts()
+            ..TargetOptions::default()
         },
     })
 }

src/librustc_target/spec/freestanding_base.rs

@@ -55,7 +55,6 @@ mod avr_gnu_base;
 mod cloudabi_base;
 mod dragonfly_base;
 mod freebsd_base;
-mod freestanding_base;
 mod fuchsia_base;
 mod haiku_base;
 mod hermit_base;