[asan] [test] Mark a new test UNSUPPORTED for MinGW targets

This test uses the MSVC/clang-cl specific -EHsc flag.

This test was recently added, in
ea12c1f.

Author: mstorsjo

compiler-rt/test/asan/TestCases/Windows/issue64990.cpp

@@ -2,6 +2,8 @@
 // RUN: %clang_cl_asan %Od %s -EHsc %Fe%t
 // RUN: not %run %t 2>&1 | FileCheck %s
 
+// UNSUPPORTED: target={{.*-windows-gnu}}
+
 char buff1[6] = "hello";
 char buff2[6] = "hello";