Fix C++11 build with Arm Compiler 6

Currently there are two issues which prevent building Mbed OS with
-std=gnu++11 when using Arm Compiler 6:
* NanostackRfPhys2lp.cpp contains a narrowing conversion in a braced
  initializer list
* ns_types.h includes <stdalign.h> which Arm Compiler 6 currently
  does not provide

This patch fixes both issues. The first one is fixed by changing the
underlying type of the corresponding enumeration when the code is
compiled as C++11. The second issue is worked around by avoiding the
use of <stdalign.h> header for Arm Compiler versions prior to 6.12.

components/802.15.4_RF/stm-s2lp-rf-driver/source/s2lpReg.h

@@ -301,7 +301,11 @@ typedef enum {
     S2LP_STATE_SYNTH_SETUP  = 0x50
 } s2lp_states_e;
 
+#if defined __cplusplus && __cplusplus >= 201103
+typedef enum : uint8_t {
+#else
 typedef enum {
+#endif
     S2LP_CMD_TX = 0x60,
     S2LP_CMD_RX,
     S2LP_CMD_READY,

features/frameworks/nanostack-libservice/mbed-client-libservice/ns_types.h

@@ -121,7 +121,15 @@ typedef int_fast32_t int_fast24_t;
 #define alignas(n) __align(n)
 #define __alignas_is_defined 1
 #elif (defined __STDC_VERSION__ && __STDC_VERSION__ >= 201112L) || (defined __cplusplus && __cplusplus >= 201103L)
+# if defined __ARMCC_VERSION && __ARMCC_VERSION < 6120000
+    /* Workaround for Arm Compiler versions prior to 6.12 */
+#   if !defined __cplusplus
+#     define alignas _Alignas
+#   endif
+#   define __alignas_is_defined 1
+# else
 #   include <stdalign.h>
+# endif
 #elif defined __GNUC__
 #define alignas(n) __attribute__((__aligned__(n)))
 #define __alignas_is_defined 1