UnitTestFrameworkPkg: SampleUnitTestGenerateException: Fix for AArch64

AArch64 does not generate divide-by-zero exceptions like x86. Yet the
current generate exception test is using this operation to attempt
triggering exceptions.

This change abstracted the exception generation logic into per-arch
files and use undefined instruction to trigger AArch64 exceptions.

Signed-off-by: Kun Qin <kun.qin@microsoft.com>