ANDROID: Define common set of cflags for kselftests

Use a common set of cflags (-D_GNU_SOURCE= and -pthread) for all
kselftests.  This should avoid _GNU_SOURCE issues going forward, make it
easier to change copts that apply to all tests, and eliminate some
configuration duplication.

Bug: 339526723
Signed-off-by: Edward Liaw <edliaw@google.com>
(cherry picked from https://android-review.googlesource.com/q/commit:c932d348c5f7de8844ae8d32c49f2a1663fa887f)
Merged-In: If0f9f4a2aaa397c1d0cb1df8ea9fd1e3bec19149
Change-Id: If0f9f4a2aaa397c1d0cb1df8ea9fd1e3bec19149