tests: gdbstub: minor refactor to the testcase.yaml

Change the `platform_allow` to multi-line format so that newly
supported archs/boards can be added as a new line.

Refactor the tags out to `common`, and add
`CONFIG_ARCH_HAS_GDBSTUB` filter

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>
(cherry picked from commit b97d97ff)