[gdb/testsuite] Improve gdb.arch/arm-pthread_cond_timedwait-bt.exp

I noticed in test-case gdb.arch/arm-pthread_cond_timedwait-bt.exp that
prepare_for_testing is used, followed by a clean_restart.

This calls clean_restart twice in a row.

Fix this by using build_executable instead.

Also, I noticed that the test-case requires an SVC instruction, so add a
require to limit the test-case to supported architectures.

While we're at it, run M-x indent-region in emacs to fix indentation.

Tested on x86_64-linux.