[Fuzzer][Test-Only] Re-enable fuzzer-ubsan.test on Darwin (llvm#165403)

This test is now XPASSing due to a linker update on the platform.

This patch removes the XFAIL from the test.

rdar://163149345

Author: DanBlackwell

compiler-rt/test/fuzzer/fuzzer-ubsan.test

@@ -1,6 +1,3 @@
-// This test currently fails to compile on green.lab.llvm.org (arm)
-// XFAIL: system-darwin && target=arm{{.*}}
-
 RUN: %cpp_compiler -fsanitize=undefined -fno-sanitize-recover=all %S/SignedIntOverflowTest.cpp -o %t-SignedIntOverflowTest-Ubsan
 RUN: not %run %t-SignedIntOverflowTest-Ubsan 2>&1 | FileCheck %s
 CHECK: runtime error: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'