Remove XLNX_PLATFORM Flag from libmetal

[tnmysh]

XLNX_PLATFORM flag is not passed to libmetal clients such as OpenAMP, they need to define it to compile with current libmetal. Instead fix this in libmetal in a way where libmetal clients do not need to define this flag.

[github-actions]

This issue has been marked as a stale issue because it has been open (more than) 45 days with no activity.

[nathalie-ckc]

Discussed in OpenAMP System Reference call 2/21/2024:
Xilinx-specific BSP code in libmetal -> need to implement libmetal API into Xilinx BSP.
Target October 2024 release.

[github-actions]

This issue has been marked as a stale issue because it has been open (more than) 45 days with no activity.