[libunwind] Use __builtin_alloca to avoid missing include

Differential Revision: https://reviews.llvm.org/D149635

Author: xbjfk

libunwind/src/config.h

@@ -127,7 +127,7 @@
 #if defined(_LIBUNWIND_REMEMBER_STACK_ALLOC) || defined(__APPLE__) ||          \
     defined(__linux__) || defined(__ANDROID__) || defined(__MINGW32__) ||      \
     defined(_LIBUNWIND_IS_BAREMETAL)
-#define _LIBUNWIND_REMEMBER_ALLOC(_size) alloca(_size)
+#define _LIBUNWIND_REMEMBER_ALLOC(_size) __builtin_alloca(_size)
 #define _LIBUNWIND_REMEMBER_FREE(_ptr)                                         \
   do {                                                                         \
   } while (0)