tests: gen_isr_table: Add barriers after triggering the irq

Fixes the gen_isr_table kernel test on mimxrt1050_evk by using data and
instruction synchronization barriers instead of disabling compiler
optimization on arm platforms. According to [1] section 4.5, "if a
pended interrupt request needs to be recognized immediately after being
enabled in the NVIC, add a DSB instruction and then an ISB instruction"

[1] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dai0321a/BIHJDAAE.html



Signed-off-by: Maureen Helm <maureen.helm@nxp.com>