cpu/cc2538: add eh_frame section to the linkerscript

Fixes linker errors on some toolchains, for example:
  ld: foo.elf section `.eh_frame' will not fit in region `cca'
  ld: region `cca' overflowed by 40 bytes

cpu/cc2538/cc2538_linkerscript.ld

@@ -96,6 +96,13 @@ SECTIONS
     } > rom
     PROVIDE_HIDDEN (__exidx_end = .);
 
+    /* exception handling */
+    . = ALIGN(4);
+    .eh_frame :
+    {
+        KEEP (*(.eh_frame))
+    } > rom
+
     . = ALIGN(4);
     _etext = .;