unconditonally define alloca as __builtin_alloca

This enables alternative compilers, which may not define __GNUC__,
to implement alloca, which is still fairly widely used.

This is similar to how stdarg.h already works in musl; compilers must
implement __builtin_va_arg, there is no fallback definition.

include/alloca.h

@@ -10,9 +10,7 @@ extern "C" {
 
 void *alloca(size_t);
 
-#ifdef __GNUC__
 #define alloca __builtin_alloca
-#endif
 
 #ifdef __cplusplus
 }