NRF: Migrate ARM Compiler 5 feature re-locate zero initialized variables

ARM Compiler 6 does not support placing a zero-initialized variable in a
zero-initialized section with a given name.
The migration path for this feature is what is as done in this commit.
The name of the section is what comes after `.bss.` (`nvictable` in thi case).

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/reloc_vector_table.c

@@ -48,7 +48,7 @@
 #endif
 
 #if defined(__CC_ARM) || (defined(__ARMCC_VERSION) && (__ARMCC_VERSION >= 6010050))
- __attribute__ ((section(".bss.nvictable"),zero_init))
+ __attribute__ ((section(".bss.nvictable")))
  uint32_t nrf_dispatch_vector[NVIC_NUM_VECTORS];
 #elif defined(__GNUC__)
  __attribute__ ((section(".nvictable")))