tccasm: define __ASSEMBLER__ for .S files, like gcc does

libtcc.c

@@ -1080,6 +1080,12 @@ ST_FUNC int tcc_add_file_internal(TCCState *s1, const char *filename, int flags)
     if (ext[0])
         ext++;
 
+#ifdef CONFIG_TCC_ASM
+    /* if .S file, define __ASSEMBLER__ like gcc does */
+    if (!strcmp(ext, "S"))
+        tcc_define_symbol(s1, "__ASSEMBLER__", NULL);
+#endif
+
     /* open the file */
     ret = tcc_open(s1, filename);
     if (ret < 0) {

tests/asmtest.S

@@ -564,3 +564,7 @@ int $0x10
     pushl $1
     pushw $1
     push $1
+
+#ifdef __ASSEMBLER__ // should be defined, for S files
+    inc %eax
+#endif