[libc] Fix building the RPC server with LIBC_NAMESPACE (#65642)

A recent patch required the implementation to define `LIBC_NAMESPACE`.
For GPU offloading we provide a static library whose internal
implementation relies on the `libc` headers. This is a separate library
that is constructed during the "bootstrap" phase. This patch moves the
definition of the `LIBC_NAMESPACE` CMake variable up so its available
during bootstrapping and adds it to the definition of the RPC server.

libc/CMakeLists.txt

@@ -19,6 +19,11 @@ set(LIBC_BUILD_DIR ${CMAKE_CURRENT_BINARY_DIR})
 
 set(LIBC_ENABLE_USE_BY_CLANG OFF CACHE BOOL "Whether or not to place libc in a build directory findable by a just built clang")
 
+# Defining a global namespace to enclose all libc functions.
+set(LIBC_NAMESPACE "__llvm_libc_${LLVM_VERSION_MAJOR}_${LLVM_VERSION_MINOR}_${LLVM_VERSION_PATCH}_${LLVM_VERSION_SUFFIX}"
+  CACHE STRING "The namespace to use to enclose internal implementations. Must start with '__llvm_libc'."
+)
+
 if(LLVM_LIBC_FULL_BUILD OR LIBC_GPU_BUILD OR LIBC_GPU_ARCHITECTURES)
   if(NOT LIBC_HDRGEN_EXE)
     # We need to set up hdrgen first since other targets depend on it.
@@ -49,11 +54,6 @@ option(LIBC_CMAKE_VERBOSE_LOGGING
 # Path libc/scripts directory.
 set(LIBC_BUILD_SCRIPTS_DIR "${LIBC_SOURCE_DIR}/utils/build_scripts")
 
-# Defining a global namespace to enclose all libc functions.
-set(LIBC_NAMESPACE "__llvm_libc_${LLVM_VERSION_MAJOR}_${LLVM_VERSION_MINOR}_${LLVM_VERSION_PATCH}_${LLVM_VERSION_SUFFIX}"
-  CACHE STRING "The namespace to use to enclose internal implementations. Must start with '__llvm_libc'."
-)
-
 if(NOT LIBC_NAMESPACE MATCHES "^__llvm_libc")
   message(FATAL_ERROR "Invalid LIBC_NAMESPACE. Must start with '__llvm_libc' was '${LIBC_NAMESPACE}'")
 endif()

libc/utils/gpu/server/CMakeLists.txt

@@ -8,6 +8,8 @@ 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>)
+target_compile_definitions(llvmlibc_rpc_server PUBLIC
+                           LIBC_NAMESPACE=${LIBC_NAMESPACE})
 
 # Install the server and associated header.
 install(FILES ${CMAKE_CURRENT_SOURCE_DIR}/rpc_server.h