Fix gcc10 warnings

i386-gen.c:
- load/gen_opf: set v1.sym to NULL

lib/Makefile:
- Add -gstabs -fno-omit-frame-pointer -Wno-unused-function -Wno-unused-variable

lib/bt-log.c:
- tcc_backtrace: Prevent __builtin_frame_address warning

tccgen.c:
- struct_layout: Set t.t to VT_BYTE
- default_debug: Use octal instead of -1 to make size_t work

tccpp.c:
- tal_realloc_impl: Only memcpy when p set

x86_64-gen.c:
- gen_bounds_epilog: Do not save/restore rcx (not caller/callee saved)
                     This also made stack not aligned to 16 bytes.

lib/bt-log.c

@@ -13,6 +13,15 @@ int (*__rt_error)(void*, void*, const char *, va_list);
 # define DLL_EXPORT
 #endif
 
+#if defined(__GNUC__) && (__GNUC__ >= 6)
+/*
+ * At least gcc 6.2 complains when __builtin_frame_address is used with
+ * nonzero argument.
+ */
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wframe-address"
+#endif
+
 DLL_EXPORT int tcc_backtrace(const char *fmt, ...)
 {
     va_list ap;
@@ -35,3 +44,7 @@ DLL_EXPORT int tcc_backtrace(const char *fmt, ...)
     }
     return ret;
 }
+
+#if defined(__GNUC__) && (__GNUC__ >= 6)
+#pragma GCC diagnostic pop
+#endif