[libc] [llvm] [libc] Make RPC server handling header only (PR #131205)

[Tulio Magno Quites Machado Filho via llvm-commits]

tuliom wrote:

This commit broke the build on s390x because libc prevents building on this architecture:
```
In file included from /builddir/build/BUILD/llvm-21.0.0_pre20250314.gf23bbf69f6ac7e-build/llvm-project-f23bbf69f6ac7ec8bcd0435419c1998f4683cc13/offload/plugins-nextgen/common/src/RPC.cpp:18:
In file included from /builddir/build/BUILD/llvm-21.0.0_pre20250314.gf23bbf69f6ac7e-build/llvm-project-f23bbf69f6ac7ec8bcd0435419c1998f4683cc13/llvm/cmake/modules/../../../libc/shared/rpc_server.h:12:
In file included from /builddir/build/BUILD/llvm-21.0.0_pre20250314.gf23bbf69f6ac7e-build/llvm-project-f23bbf69f6ac7ec8bcd0435419c1998f4683cc13/llvm/cmake/modules/../../../libc/src/__support/RPC/rpc_server.h:38:
In file included from /builddir/build/BUILD/llvm-21.0.0_pre20250314.gf23bbf69f6ac7e-build/llvm-project-f23bbf69f6ac7ec8bcd0435419c1998f4683cc13/llvm/cmake/modules/../../../libc/src/stdio/printf_core/converter.h:15:
In file included from /builddir/build/BUILD/llvm-21.0.0_pre20250314.gf23bbf69f6ac7e-build/llvm-project-f23bbf69f6ac7ec8bcd0435419c1998f4683cc13/llvm/cmake/modules/../../../libc/src/stdio/printf_core/strerror_converter.h:15:
In file included from /builddir/build/BUILD/llvm-21.0.0_pre20250314.gf23bbf69f6ac7e-build/llvm-project-f23bbf69f6ac7ec8bcd0435419c1998f4683cc13/llvm/cmake/modules/../../../libc/src/stdio/printf_core/int_converter.h:19:
In file included from /builddir/build/BUILD/llvm-21.0.0_pre20250314.gf23bbf69f6ac7e-build/llvm-project-f23bbf69f6ac7ec8bcd0435419c1998f4683cc13/llvm/cmake/modules/../../../libc/src/stdio/printf_core/writer.h:17:
/builddir/build/BUILD/llvm-21.0.0_pre20250314.gf23bbf69f6ac7e-build/llvm-project-f23bbf69f6ac7ec8bcd0435419c1998f4683cc13/llvm/cmake/modules/../../../libc/src/string/memory_utils/inline_memset.h:34:2: error: "Unsupported architecture"
   34 | #error "Unsupported architecture"
      |  ^
```

https://github.com/llvm/llvm-project/pull/131205