[compiler-rt][test] Add shared_unwind requirement

When using a static libunwind, the check_memcpy.c can fail because it checks
that tsan intercepted all memcpy/memmoves in the final binary. Though if the
static libunwind is not instrumented, then this will fail because it may contain
regular memcpy/memmoves.

This adds a new REQUIRES check for ensuring that this test won't run unless a
dynamic libunwind.so is provided.

Differential Revision: https://reviews.llvm.org/D111194