[LibOS] Compile openmp test properly with Clang

Make sure we still use our patched libgomp.

Signed-off-by: Paweł Marczewski <pawel@invisiblethingslab.com>

LibOS/shim/test/regression/Makefile

@@ -135,7 +135,6 @@ CFLAGS-debug = -g3
 CFLAGS-debug_regs-x86_64 = -g3
 CFLAGS-exec_same = -pthread
 CFLAGS-shared_object = -fPIC -pie
-CFLAGS-openmp = -fopenmp
 CFLAGS-multi_pthread = -pthread
 CFLAGS-exit_group = -pthread
 CFLAGS-abort_multithread = -pthread
@@ -157,6 +156,12 @@ LDLIBS-attestation += ../../../../common/src/crypto/mbedtls/install/lib/libmbedc
 CFLAGS-fp_multithread += -pthread -fno-builtin  # see comment in the test's source
 LDLIBS-fp_multithread += -lm
 
+ifeq ($(findstring clang,$(CC)),clang)
+CFLAGS-openmp = -fopenmp=libgomp
+else
+CFLAGS-openmp = -fopenmp
+endif
+
 proc_common: proc_common.o dump.o
 	$(call cmd,cmulti)