PHY: Initialize pucch3_w_n_oc_5 using precomputed constants when compiled with clang

Fixes the following clang-7's error

srsLTE/lib/src/phy/phch/pucch.c:307:9: error: initializer element is not a compile-time constant
    {1, cexpf(I * 2 * M_PI / 5), cexpf(I * 4 * M_PI / 5), cexpf(I * 6 * M_PI / 5), cexpf(I * 8 * M_PI / 5)},
        ^~~~~~~~~~~~~~~~~~~~~~~

Add SRSLTE_PUCCH_FORMAT_3 in the pucch_test

lib/src/phy/phch/pucch.c

@@ -302,6 +302,31 @@ static const float w_n_oc[2][3][4] = {
 
 };
 
+#if defined(__clang__)
+
+/* Precomputed constants printed with printf %a specifier (hexadecimal notation) for maximum precision
+ *    cf_t val = cexpf(I * 2 * M_PI / 5));
+ *    printf("%a + %aI\n", str, creal(val), cimag(val));
+ *
+ * clang expects compile-time contant expressions in the initializer list and using cexpf results
+ * in the following error
+ *    error: initializer element is not a compile-time constant
+ */
+static const cf_t cexpf_i_2_mpi_5 = 0x1.3c6ef2p-2 + 0x1.e6f0e2p-1I;
+static const cf_t cexpf_i_4_mpi_5 = -0x1.9e377cp-1 + 0x1.2cf22ep-1I;
+static const cf_t cexpf_i_6_mpi_5 = -0x1.9e3778p-1 + -0x1.2cf234p-1I;
+static const cf_t cexpf_i_8_mpi_5 = 0x1.3c6efcp-2 + -0x1.e6f0ep-1I;
+
+static cf_t pucch3_w_n_oc_5[5][5] = {
+    {1, 1, 1, 1, 1},
+    {1, cexpf_i_2_mpi_5, cexpf_i_4_mpi_5, cexpf_i_6_mpi_5, cexpf_i_8_mpi_5},
+    {1, cexpf_i_4_mpi_5, cexpf_i_8_mpi_5, cexpf_i_2_mpi_5, cexpf_i_6_mpi_5},
+    {1, cexpf_i_6_mpi_5, cexpf_i_2_mpi_5, cexpf_i_8_mpi_5, cexpf_i_4_mpi_5},
+    {1, cexpf_i_8_mpi_5, cexpf_i_6_mpi_5, cexpf_i_4_mpi_5, cexpf_i_2_mpi_5},
+};
+
+#else // defined(__clang__)
+
 static const cf_t pucch3_w_n_oc_5[5][5] = {
     {1, 1, 1, 1, 1},
     {1, cexpf(I * 2 * M_PI / 5), cexpf(I * 4 * M_PI / 5), cexpf(I * 6 * M_PI / 5), cexpf(I * 8 * M_PI / 5)},
@@ -309,6 +334,8 @@ static const cf_t pucch3_w_n_oc_5[5][5] = {
     {1, cexpf(I * 6 * M_PI / 5), cexpf(I * 2 * M_PI / 5), cexpf(I * 8 * M_PI / 5), cexpf(I * 4 * M_PI / 5)},
     {1, cexpf(I * 8 * M_PI / 5), cexpf(I * 6 * M_PI / 5), cexpf(I * 4 * M_PI / 5), cexpf(I * 2 * M_PI / 5)}};
 
+#endif // defined(__clang__)
+
 static const cf_t pucch3_w_n_oc_4[4][4] = {{+1, +1, +1, +1}, {+1, -1, +1, -1}, {+1, +1, -1, -1}, {+1, -1, -1, +1}};
 
 static uint32_t get_N_sf(srslte_pucch_format_t format, uint32_t slot_idx, bool shortened)

lib/src/phy/phch/test/pucch_test.c

@@ -177,7 +177,7 @@ int main(int argc, char **argv) {
   ZERO_OBJECT(ul_sf);
 
   srslte_pucch_format_t format;
-  for (format=0;format<=SRSLTE_PUCCH_FORMAT_2B;format++) {
+  for (format = 0; format <= SRSLTE_PUCCH_FORMAT_3; format++) {
     for (uint32_t d=1;d<=3;d++) {
       for (uint32_t ncs=0;ncs<8;ncs+=d) {
         for (uint32_t n_pucch=1;n_pucch<130;n_pucch+=50) {
@@ -208,6 +208,7 @@ int main(int argc, char **argv) {
               break;
             case SRSLTE_PUCCH_FORMAT_1B:
             case SRSLTE_PUCCH_FORMAT_2B:
+            case SRSLTE_PUCCH_FORMAT_3:
               uci_data.value.ack.ack_value[0] = 1;
               uci_data.value.ack.ack_value[1] = 1;
               uci_data.cfg.ack[0].nof_acks    = 2;