[libc] Make the RPC server target always available

This patch makes sure that we always build the RPC server. The proposed
used for this is to begin integrating this server implementation into
`libomptarget`. That requires that we build this server ahead of time
when using a `LLVM_ENABLE_PROJECTS` build. Make a few tweaks to ensure
that the GCC compiler which may be used for this build doesn't complain.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D154105

libc/CMakeLists.txt

@@ -35,6 +35,11 @@ if(("libc" IN_LIST LLVM_ENABLE_RUNTIMES AND NOT LLVM_RUNTIMES_BUILD) OR
   # When libc is build as part of the runtimes/bootstrap build's CMake run, we
   # only need to build the host tools to build the libc. So, we just do enough
   # to build libc-hdrgen and return.
+
+  # Always make the RPC server availible to other projects for GPU mode.
+  if(LIBC_GPU_BUILD OR LIBC_GPU_ARCHITECTURES)
+    add_subdirectory(utils/gpu/server)
+  endif()
   return()
 endif()
 

libc/utils/gpu/server/CMakeLists.txt

@@ -5,6 +5,10 @@ target_include_directories(llvmlibc_rpc_server PRIVATE ${LIBC_SOURCE_DIR})
 target_include_directories(llvmlibc_rpc_server PUBLIC ${LIBC_SOURCE_DIR}/include)
 target_include_directories(llvmlibc_rpc_server PUBLIC ${CMAKE_CURRENT_SOURCE_DIR})
 
+# Ignore unsupported clang attributes if we're using GCC.
+target_compile_options(llvmlibc_rpc_server PUBLIC
+                       $<$<CXX_COMPILER_ID:GNU>:-Wno-attributes>)
+
 # Install the server and associated header.
 install(FILES ${CMAKE_CURRENT_SOURCE_DIR}/rpc_server.h
         DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/gpu-none-llvm/

libc/utils/gpu/server/rpc_server.h

@@ -22,7 +22,7 @@ const uint64_t RPC_MAXIMUM_PORT_COUNT = 64;
 
 /// The symbol name associated with the client for use with the LLVM C library
 /// implementation.
-static const char *rpc_client_symbol_name = "__llvm_libc_rpc_client";
+inline const char *rpc_client_symbol_name = "__llvm_libc_rpc_client";
 
 /// status codes.
 typedef enum {