cortex-m0: Ensure .bss.slow section is cleared.

The .bss.slow input section needs to be included before __bss_end,
otherwise it won't get cleared to zero.

BUG=None
BRANCH=None
TEST=Build jerry.  Verify that __bss_end is at the end of the .bss.slow
section.
TEST=make -j buildall tests

Change-Id: I41d028ee166d05b34a889499cdcb0254341be1b6
Signed-off-by: Aseda Aboagye <aaboagye@google.com>
Reviewed-on: https://chromium-review.googlesource.com/312404
Commit-Ready: Aseda Aboagye <aaboagye@chromium.org>
Tested-by: Aseda Aboagye <aaboagye@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>

core/cortex-m0/ec.lds.S

@@ -181,6 +181,7 @@ SECTIONS
         *(.bss.system_stack)
 	/* Rest of .bss takes care of its own alignment */
         *(.bss)
+        *(.bss.slow)
         . = ALIGN(4);
         __bss_end = .;
     } > IRAM