[libc++] Update XFAIL for layout_stride/assert.conversion.pass.cpp in debug...

[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 
58780b81 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 <maryammo@ca.ibm.com>