Add optional backtrace implementation for validation layer

[kswiecicki]

There's an open PR that adds a simple leak-checking mechanism to the validation layer. Leak-checking mechanism searches for unreleased references at the time of the urTearDown() call and provides a backtrace of the time each unreleased record was first recorded.

Default implementation on Linux uses the <execinfo.h> to provide a backtrace that might not be entirely useful. It looks like this:

<VALIDATION>[ERROR]: /home/kswiecic/dev/sycl_workspace/unified-runtime/build/lib/libur_loader.so.0(+0x62f4e) [0x7f6d71addf4e]
<VALIDATION>[ERROR]: /home/kswiecic/dev/sycl_workspace/unified-runtime/build/lib/libur_loader.so.0(urContextCreate+0x56) [0x7f6d71ad0ed6]
<VALIDATION>[ERROR]: /home/kswiecic/dev/sycl_workspace/unified-runtime/build/bin/validation_test-leaks_mt(+0x11258) [0x558370258258]
<VALIDATION>[ERROR]: /home/kswiecic/dev/sycl_workspace/unified-runtime/build/bin/validation_test-leaks_mt(+0x74f8b) [0x5583702bbf8b]
<VALIDATION>[ERROR]: /home/kswiecic/dev/sycl_workspace/unified-runtime/build/bin/validation_test-leaks_mt(+0x5802a) [0x55837029f02a]
<VALIDATION>[ERROR]: /home/kswiecic/dev/sycl_workspace/unified-runtime/build/bin/validation_test-leaks_mt(+0x390e3) [0x5583702800e3]
<VALIDATION>[ERROR]: /home/kswiecic/dev/sycl_workspace/unified-runtime/build/bin/validation_test-leaks_mt(+0x39c70) [0x558370280c70]
<VALIDATION>[ERROR]: /home/kswiecic/dev/sycl_workspace/unified-runtime/build/bin/validation_test-leaks_mt(+0x3a4bd) [0x5583702814bd]
<VALIDATION>[ERROR]: /home/kswiecic/dev/sycl_workspace/unified-runtime/build/bin/validation_test-leaks_mt(+0x4aeb4) [0x558370291eb4]
<VALIDATION>[ERROR]: /home/kswiecic/dev/sycl_workspace/unified-runtime/build/bin/validation_test-leaks_mt(+0x77beb) [0x5583702bebeb]
<VALIDATION>[ERROR]: /home/kswiecic/dev/sycl_workspace/unified-runtime/build/bin/validation_test-leaks_mt(+0x5a47a) [0x5583702a147a]
<VALIDATION>[ERROR]: /home/kswiecic/dev/sycl_workspace/unified-runtime/build/bin/validation_test-leaks_mt(+0x4aa0b) [0x558370291a0b]
<VALIDATION>[ERROR]: /home/kswiecic/dev/sycl_workspace/unified-runtime/build/bin/validation_test-leaks_mt(+0x232a1) [0x55837026a2a1]
<VALIDATION>[ERROR]: /home/kswiecic/dev/sycl_workspace/unified-runtime/build/bin/validation_test-leaks_mt(+0x2327d) [0x55837026a27d]
<VALIDATION>[ERROR]: /lib/x86_64-linux-gnu/libc.so.6(+0x29d90) [0x7f6d71541d90]
<VALIDATION>[ERROR]: /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0x80) [0x7f6d71541e40]

It would be beneficial to provide an alternative backtrace implementation that depends on an external library to provide an unmangled backtrace, e.g. libbacktrace, libunwind. Backtrace implementation could be selected at the compile time based on the provided option eg. -DVALIDATION_BACKTRACE_LIBUNWIND.

Mentioned PR: #318.