[gn build] Build win asan libraries against MutliThreadedDLL

win asan only supports /MD. Without this, the asan runtime cannot intercept functions.

Author: aeubanks

llvm/utils/gn/secondary/compiler-rt/lib/asan/BUILD.gn

@@ -68,6 +68,7 @@ if (current_toolchain == host_toolchain) {
     if (current_os != "win") {
       cflags += [ "-ftls-model=initial-exec" ]
     } else {
+      cflags += [ "/MD" ]
       ldflags = [ "/OPT:NOICF" ]
     }