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 this
case).
Also simplify pre-processor directive to test for Arm compiler.

Author: hugueskamba

targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/reloc_vector_table.c

@@ -47,8 +47,8 @@
 #include "nrf_dfu_mbr.h"
 #endif
 
-#if defined(__CC_ARM) || (defined(__ARMCC_VERSION) && (__ARMCC_VERSION >= 6010050))
-    __attribute__ ((section(".bss.nvictable"),zero_init))
+#if defined(__ARMCC_VERSION)
+    __attribute__ ((section(".bss.nvictable")))
     uint32_t nrf_dispatch_vector[NVIC_NUM_VECTORS];
 #elif defined(__GNUC__)
     __attribute__ ((section(".nvictable")))