mpy-cross/main: Don't set a default native architecture.

If `-march` isn't set then it means the user hasn't thought about it, or in
the case of freezing, MPY_CROSS_FLAGS isn't set.  It's almost certainly
going to lead to problems, as there's no reason why the host architecture
is likely to be the right choice.

Compiling regular Python code is unaffected, but if `@native`/`@viper` is
used, the compiler will raise `SyntaxError: invalid arch`.

For situations where you explicitly want to use the host architecture (e.g.
for running tests on the unix port), added -march=host that keeps the old
behavior.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

mpy-cross/main.c

@@ -203,19 +203,9 @@ MP_NOINLINE int main_(int argc, char **argv) {
 
     // set default compiler configuration
     mp_dynamic_compiler.small_int_bits = 31;
-    #if defined(__i386__)
-    mp_dynamic_compiler.native_arch = MP_NATIVE_ARCH_X86;
-    mp_dynamic_compiler.nlr_buf_num_regs = MICROPY_NLR_NUM_REGS_X86;
-    #elif defined(__x86_64__)
-    mp_dynamic_compiler.native_arch = MP_NATIVE_ARCH_X64;
-    mp_dynamic_compiler.nlr_buf_num_regs = MAX(MICROPY_NLR_NUM_REGS_X64, MICROPY_NLR_NUM_REGS_X64_WIN);
-    #elif defined(__arm__) && !defined(__thumb2__)
-    mp_dynamic_compiler.native_arch = MP_NATIVE_ARCH_ARMV6;
-    mp_dynamic_compiler.nlr_buf_num_regs = MICROPY_NLR_NUM_REGS_ARM_THUMB_FP;
-    #else
+    // don't support native emitter unless -march is specified
     mp_dynamic_compiler.native_arch = MP_NATIVE_ARCH_NONE;
     mp_dynamic_compiler.nlr_buf_num_regs = 0;
-    #endif
 
     const char *input_file = NULL;
     const char *output_file = NULL;
@@ -292,6 +282,20 @@ MP_NOINLINE int main_(int argc, char **argv) {
                 } else if (strcmp(arch, "xtensawin") == 0) {
                     mp_dynamic_compiler.native_arch = MP_NATIVE_ARCH_XTENSAWIN;
                     mp_dynamic_compiler.nlr_buf_num_regs = MICROPY_NLR_NUM_REGS_XTENSAWIN;
+                } else if (strcmp(arch, "host") == 0) {
+                    #if defined(__i386__)
+                    mp_dynamic_compiler.native_arch = MP_NATIVE_ARCH_X86;
+                    mp_dynamic_compiler.nlr_buf_num_regs = MICROPY_NLR_NUM_REGS_X86;
+                    #elif defined(__x86_64__)
+                    mp_dynamic_compiler.native_arch = MP_NATIVE_ARCH_X64;
+                    mp_dynamic_compiler.nlr_buf_num_regs = MAX(MICROPY_NLR_NUM_REGS_X64, MICROPY_NLR_NUM_REGS_X64_WIN);
+                    #elif defined(__arm__) && !defined(__thumb2__)
+                    mp_dynamic_compiler.native_arch = MP_NATIVE_ARCH_ARMV6;
+                    mp_dynamic_compiler.nlr_buf_num_regs = MICROPY_NLR_NUM_REGS_ARM_THUMB_FP;
+                    #else
+                    mp_printf(&mp_stderr_print, "unable to determine host architecture for -march=host\n");
+                    exit(1);
+                    #endif
                 } else {
                     return usage(argv);
                 }