[libcxx][test] Explain picolib unsupported in sort.pass.cpp

Summary: This is not a hidden bug, it's just a very slow test under emulation.

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Differential Revision: https://phabricator.intern.facebook.com/D60250711

Author: DavidSpickett

libcxx/test/std/algorithms/alg.sorting/alg.sort/sort/sort.pass.cpp

@@ -6,7 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
-// This test appears to hang with picolibc & qemu.
+// This test did pass but is very slow when run using qemu. ~7 minutes on a
+// Neoverse N1 (AArch64) server core.
 // UNSUPPORTED: LIBCXX-PICOLIBC-FIXME
 
 // <algorithm>