Warn about soft float ABI not being supported

For ARM target, tcc uses the soft float ABI when not asked to use hard
float ABI. This means machine without a VFP co-processor generate code
that they cannot run. This commit add a warning for such cases.

arm-gen.c

@@ -186,6 +186,9 @@ ST_FUNC void arm_init(struct TCCState *s)
     func_double_type.ref = sym_push(SYM_FIELD, &double_type, FUNC_CDECL, FUNC_OLD);
 
     float_abi = s->float_abi;
+#ifndef TCC_ARM_HARDFLOAT
+    tcc_warning("soft float ABI currently not supported: default to softfp");
+#endif
 }
 #else
 #define func_float_type func_old_type