--legacyalign, --no_legacyalign are deprecated from ARMC6 compiler, in order to
remove deprecated flags all linker files (GCC and IAR as well to have uniformity)
should strictly align to 8-byte boundary
Set the ISR stack to be 1KB. https://github.com/ARMmbed/mbed-os/pull/7238
Set the heap size to 3KB(2KB + overhead + spare) so that atleast 2KB free ram is
available for testing.
With dynamic heap size, explicit size is not required. IAR 7.8 supports
static heap, hence the change is needed in IAR linker files.