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
Merge pull request #2122 from locicontrols/eh_frame

cpu/cc2538: add the exception handling frame section "eh_frame" to the linker script.