nrf: ld: add ARM.exidx to output image

With the WDT changes, building Circuit Python results in the following error:

/opt/gcc-arm-none-eabi-9-2019-q4-major/bin/../lib/gcc/arm-none-eabi/9.2.1/../../../../arm-none-eabi/bin/ld: section .ARM.exidx LMA [00000000000621c8,00000000000621cf] overlaps section .data LMA [00000000000621c8,0000000000062383]

This is because unwinding data is getting generated, but has nowhere to go.
Re-enable this data in the linker script so it is saved.

Signed-off-by: Sean Cross <sean@xobs.io>

ports/nrf/boards/common.template.ld

@@ -70,7 +70,6 @@ SECTIONS
         _etext = .;        /* define a global symbol at end of code */
     } >FLASH_FIRMWARE
 
-    /*
     .ARM.extab :
     {
         *(.ARM.extab* .gnu.linkonce.armextab.*)
@@ -82,7 +81,6 @@ SECTIONS
         *(.ARM.exidx*)
         __exidx_end = .;
     } >FLASH_FIRMWARE
-    */
 
     /* used by the startup to initialize data */
     _sidata = .;