run-tests: set python stdio encoding to utf-8

.. unsetting this causes python3 to fail to start at all,
stating "Fatal Python error: Failed to initialize Windows random API
(CryptoGen)".  See https://bugs.python.org/issue20614

py/argcheck.c

@@ -32,7 +32,9 @@
 void mp_arg_check_num(size_t n_args, size_t n_kw, size_t n_args_min, size_t n_args_max, bool takes_kw) {
     // NOTE(tannewt): This prevents this function from being optimized away.
     // Without it, functions can crash when reading invalid args.
+#ifdef __GNUC__
     __asm volatile ("");
+#endif
     // TODO maybe take the function name as an argument so we can print nicer error messages
 
     if (n_kw && !takes_kw) {

py/nlr.h

@@ -55,6 +55,7 @@ struct _nlr_buf_t {
 #elif defined(__xtensa__)
     void *regs[10];
 #else
+    #undef MICROPY_NLR_SETJMP
     #define MICROPY_NLR_SETJMP (1)
     //#warning "No native NLR support for this arch, using setjmp implementation"
 #endif

py/persistentcode.c

@@ -124,7 +124,11 @@ STATIC size_t read_uint(mp_reader_t *reader) {
 
 STATIC qstr load_qstr(mp_reader_t *reader) {
     size_t len = read_uint(reader);
+#ifdef __GNUC__
     char str[len];
+#else
+    char *str = alloca(len);
+#endif
     read_bytes(reader, (byte*)str, len);
     qstr qst = qstr_from_strn(str, len);
     return qst;

py/stackctrl.c

@@ -29,7 +29,9 @@
 
 void mp_stack_ctrl_init(void) {
     // Force routine to not be inlined. Better guarantee than MP_NOINLINE for -flto.
+#ifdef __GNUC__
     __asm volatile ("");
+#endif
     volatile int stack_dummy;
     MP_STATE_THREAD(stack_top) = (char*)&stack_dummy;
 }
@@ -41,7 +43,9 @@ void mp_stack_set_top(void *top) {
 mp_uint_t mp_stack_usage(void) {
     // Assumes descending stack
     // Force routine to not be inlined. Better guarantee than MP_NOINLINE for -flto.
+#ifdef __GNUC__
     __asm volatile ("");
+#endif
     volatile int stack_dummy;
     return MP_STATE_THREAD(stack_top) - (char*)&stack_dummy;
 }

tests/run-tests

@@ -389,7 +389,10 @@ def run_tests(pyb, tests, args, base_path="."):
             # run CPython to work out expected output
             e = {"PYTHONPATH": os.getcwd(),
                  "PATH": os.environ["PATH"],
-                 "LANG": "en_US.UTF-8"}
+                 "LANG": "en_US.UTF-8",
+                 "PYTHONIOENCODING": "utf-8"}
+            if 'SYSTEMROOT' in os.environ:
+                e['SYSTEMROOT'] = os.environ['SYSTEMROOT']
             p = subprocess.Popen([CPYTHON3, '-B', test_file], env=e, stdout=subprocess.PIPE)
             output_expected = b''
             while p.poll() is None: