qemu-arm/mpconfigport: Define MICROPY_PY_SYS_PLATFORM.

Signed-off-by: Damien George <damien@micropython.org>

ports/qemu-arm/mpconfigport.h

@@ -34,6 +34,7 @@
 #define MICROPY_PY_IO               (1)
 #define MICROPY_PY_SYS_EXIT         (1)
 #define MICROPY_PY_SYS_MAXSIZE      (1)
+#define MICROPY_PY_SYS_PLATFORM     "qemu-arm"
 #define MICROPY_PY_UERRNO           (1)
 #define MICROPY_PY_UBINASCII        (1)
 #define MICROPY_PY_URANDOM          (1)