do not assume tco unless musttail attribute is available

although i'm not entirely happy with this, i can't think of
a better way right now.

note: the xtensa version of llvm has broken musttail.
espressif/llvm-project#73

Author: yamt

lib/insn.c

@@ -541,7 +541,7 @@ read_memarg_nocheck(const uint8_t **pp, struct memarg *arg)
 #define SAVE_STACK_PTR ctx->stack.lsize = stack - ctx->stack.p
 #define LOAD_STACK_PTR stack = &VEC_NEXTELEM(ctx->stack)
 #define ORIG_PC p0
-#if defined(TOYWASM_USE_TAILCALL)
+#if defined(TOYWASM_USE_TAILCALL) && defined(__HAVE_TAILCALL)
 #define INSN_SUCCESS __musttail return fetch_exec_next_insn(p, stack, ctx)
 #else
 #define INSN_SUCCESS INSN_SUCCESS_RETURN

lib/platform.h

@@ -4,8 +4,10 @@
 
 #if defined(__clang__) && defined(__has_attribute) && __has_attribute(musttail)
 #define __musttail __attribute__((musttail))
+#define __HAVE_TAILCALL
 #else
 #define __musttail
+#undef __HAVE_TAILCALL
 #endif
 
 #if !defined(__predict_true)