[test][compiler-rt] Mark dlsym_alloc.c as unsupported on macos (#108439)

With #106912, the test now fails on macos, e.g.

https://green.lab.llvm.org/job/llvm.org/job/clang-stage1-RA/2058/.

Author: aeubanks

compiler-rt/test/sanitizer_common/TestCases/dlsym_alloc.c

@@ -2,6 +2,8 @@
 
 // FIXME: TSAN does not use DlsymAlloc.
 // UNSUPPORTED: tsan
+// FIXME: investigate why this fails on macos
+// UNSUPPORTED: darwin
 
 #include <stdlib.h>