move va_list into core:c

core/c/c.odin

@@ -104,3 +104,13 @@ NULL           :: rawptr(uintptr(0))
 NDEBUG         :: !ODIN_DEBUG
 
 CHAR_BIT :: 8
+
+// Since there are no types in C with an alignment larger than that of
+// max_align_t, which cannot be larger than sizeof(long double) as any other
+// exposed type wouldn't be valid C, the maximum alignment possible in a
+// strictly conformant C implementation is 16 on the platforms we care about.
+// The choice of 4096 bytes for storage of this type is more than enough on all
+// relevant platforms.
+va_list :: struct #align(16) {
+	_: [4096]u8,
+}