[libc++] Update XFAIL for layout_stride/assert.conversion.pass.cpp in debug mode on PowerPC (#79169)

Adding the debug hardening modes into PowerPC target to check the
assertion messages, to fix the PPC rehl bot [1]. This is needed as 
58780b8 changed the assertion to trap in production hardening modes.

Properly fixing these assertions is tracked by #79216.

[1]: https://lab.llvm.org/buildbot/#/builders/57/builds/32354

Co-authored-by: Maryam Moghadas <[email protected]>

Author: maryammo

libcxx/test/libcxx/containers/views/mdspan/layout_stride/assert.conversion.pass.cpp

@@ -10,7 +10,7 @@
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
 // UNSUPPORTED: libcpp-hardening-mode=none
 // XFAIL: libcpp-hardening-mode=debug && availability-verbose_abort-missing
-// XFAIL: target=powerpc{{.*}}le-unknown-linux-gnu
+// XFAIL: libcpp-hardening-mode=debug && target=powerpc{{.*}}le-unknown-linux-gnu
 
 // <mdspan>