[NFC][asan] Remove -std=c++20 from test

Fails after D110215
with errors like
/usr/include/x86_64-linux-gnu/sys/types.h:33:9: error: unknown type name '__u_char'
typedef __u_char u_char;

compiler-rt/test/asan/TestCases/Posix/unpoison-alternate-stack.cpp

@@ -3,7 +3,7 @@
 
 // Don't optimize, otherwise the variables which create redzones might be
 // dropped.
-// RUN: %clangxx_asan -std=c++20 -fexceptions -O0 %s -o %t -pthread
+// RUN: %clangxx_asan -fexceptions -O0 %s -o %t -pthread
 // RUN: %run %t
 
 #include <algorithm>