testsuite: Run array54.C only for sync_int_long targets

The test case uses "atomic<int>", which fails to link on
pru-unknown-elf target due to missing __atomic_load_4 symbol.

Fix by filtering for sync_int_long effective target.  Ensured that the
test still passes for x86_64-pc-linux-gnu.

gcc/testsuite/ChangeLog:

	* g++.dg/init/array54.C: Require sync_int_long effective target.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>