Link with libc again to fix issue #25

Before this, 'cargo build -Z build-std=core --target avr-atmega328p.json' would
fail with linking errors to 'abort' and 'memcpy' symbols.

After this patch, a debug mode executable can be successfully generated
again.

Author: dylanmckay

avr-atmega328p.json

@@ -17,7 +17,7 @@
     "gcc": ["-mmcu=atmega328p"]
   },
   "late-link-args": {
-    "gcc": ["-lgcc"]
+    "gcc": ["-lgcc", "-lc"]
   },
   "target-c-int-width": "16",
   "target-endian": "little",