[libc] Repurpose LIBC_GPU_BUILD option to enable the new format (#82848)

Summary:
We previously used the `LIBC_GPU_BUILD` option to control whether or not
the GPU build was enabled. This was recently replaced with a new format
that allows treating the GPU targets more directly. However, the new
format is somewhat difficult to use for people unfamiliar with the
runtimes builds, and the removal of this option somewhat broke backward
compatibility. This patch seeks to simplify enabling the GPU build by
repurposing the old enabling option and convert it to the new interface.

Unsure what the rules are here, since this is technically a `LIBC`
option living in the LLVM location.

Author: jhuber6

libc/CMakeLists.txt

@@ -134,7 +134,6 @@ option(LLVM_LIBC_FULL_BUILD "Build and test LLVM libc as if it is the full libc"
 option(LLVM_LIBC_IMPLEMENTATION_DEFINED_TEST_BEHAVIOR "Build LLVM libc tests assuming our implementation-defined behavior" ON)
 option(LLVM_LIBC_ENABLE_LINTING "Enables linting of libc source files" OFF)
 
-option(LIBC_GPU_BUILD "Build libc for the GPU. All CPU build options will be ignored." OFF)
 set(LIBC_TARGET_TRIPLE "" CACHE STRING "The target triple for the libc build.")
 
 option(LIBC_CONFIG_PATH "The path to user provided folder that configures the build for the target system." OFF)

llvm/CMakeLists.txt

@@ -159,6 +159,18 @@ foreach(proj IN LISTS LLVM_ENABLE_RUNTIMES)
   endif()
 endforeach()
 
+# Set a shorthand option to enable the GPU build of the 'libc' project.
+option(LIBC_GPU_BUILD "Enable the 'libc' project targeting the GPU" OFF)
+if(LIBC_GPU_BUILD)
+  if(LLVM_RUNTIME_TARGETS)
+    list(APPEND LLVM_RUNTIME_TARGETS "nvptx64-nvidia-cuda" "amdgcn-amd-amdhsa")
+  else()
+    set(LLVM_RUNTIME_TARGETS "default;nvptx64-nvidia-cuda;amdgcn-amd-amdhsa")
+  endif()
+  list(APPEND RUNTIMES_nvptx64-nvidia-cuda_LLVM_ENABLE_RUNTIMES "libc")
+  list(APPEND RUNTIMES_amdgcn-amd-amdhsa_LLVM_ENABLE_RUNTIMES "libc")
+endif()
+
 set(NEED_LIBC_HDRGEN FALSE)
 if("libc" IN_LIST LLVM_ENABLE_RUNTIMES)
   set(NEED_LIBC_HDRGEN TRUE)