kernel: Kconfig: Increase the main stack size for ARM when TEST

The following testcases fail with qemu_cortex_r5 caused by main stack
overflow.
tests/kernel/workq/work_queue/kernel.workqueue
tests/ztest/base/testing.ztest.base.verbose_0_userspace

The main stack size is 512 for qemu_cortex_r5(a Cortex-A/R aarch32
platform) with CONFIG_ZTEST=y. The Cortex-M platforms are already set to
1024. Likely 512 will fail for most aarch32 platforms soon.

Fix the issue by increasing the CONFIG_MAIN_STACK_SIZE to 1024.
Also, remove 'default 1024 if TEST_ARM_CORTEX_M' since Cortex-M is no
longer an exception of default 1024.

Signed-off-by: Jaxson Han <jaxson.han@arm.com>