Add --gc-sections linker argument again

It was previously added automatically by Rust. See #167 for more info.

Author: phil-opp

tests/test_kernels/higher_half/x86_64-higher_half.json

@@ -13,6 +13,6 @@
     "disable-redzone": true,
     "features": "-mmx,-sse,+soft-float",
     "pre-link-args": {
-      "ld.lld": ["--image-base", "0xFFFF800000000000"]
+      "ld.lld": ["--image-base", "0xFFFF800000000000", "--gc-sections"]
     }
   }

x86_64-bootloader.json

@@ -5,8 +5,9 @@
     "linker": "rust-lld",
     "pre-link-args": {
         "ld.lld": [
-	    "--script=linker.ld"
-	]
+            "--script=linker.ld",
+            "--gc-sections"
+        ]
     },
     "target-endian": "little",
     "target-pointer-width": "64",
@@ -17,5 +18,5 @@
     "disable-redzone": true,
     "panic-strategy": "abort",
     "executables": true,
-	"relocation_model": "static"
+    "relocation_model": "static"
 }