[build] Check RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES for libc also (#82561)

When checking whether we need to build libc-hdrgen, we need to check
LLVM_ENABLE_RUNTIMES and RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES, just
the former is not sufficient since libc may be enabled only for certain
targets.

Author: petrhosek

libc/CMakeLists.txt

@@ -57,9 +57,21 @@ if(LLVM_LIBC_FULL_BUILD OR LIBC_GPU_BUILD OR LIBC_GPU_ARCHITECTURES)
   endif()
 endif()
 
+set(NEED_LIBC_HDRGEN FALSE)
+if(NOT LLVM_RUNTIMES_BUILD)
+  if("libc" IN_LIST LLVM_ENABLE_RUNTIMES)
+    set(NEED_LIBC_HDRGEN TRUE)
+  else()
+    foreach(_name ${LLVM_RUNTIME_TARGETS})
+      if("libc" IN_LIST RUNTIMES_${_name}_LLVM_ENABLE_RUNTIMES)
+        set(NEED_LIBC_HDRGEN TRUE)
+        break()
+      endif()
+    endforeach()
+  endif()
+endif()
 option(LIBC_HDRGEN_ONLY "Only build the 'libc-hdrgen' executable" OFF)
-if(("libc" IN_LIST LLVM_ENABLE_RUNTIMES AND NOT LLVM_RUNTIMES_BUILD) OR 
-   LIBC_HDRGEN_ONLY)
+if(LIBC_HDRGEN_ONLY OR NEED_LIBC_HDRGEN)
   # 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.
@@ -70,6 +82,7 @@ if(("libc" IN_LIST LLVM_ENABLE_RUNTIMES AND NOT LLVM_RUNTIMES_BUILD) OR
   endif()
   return()
 endif()
+unset(NEED_LIBC_HDRGEN)
 
 option(LIBC_CMAKE_VERBOSE_LOGGING
   "Log details warnings and notifications during CMake configuration." OFF)

llvm/CMakeLists.txt

@@ -168,7 +168,18 @@ foreach(proj IN LISTS LLVM_ENABLE_RUNTIMES)
   endif()
 endforeach()
 
-if ("libc" IN_LIST LLVM_ENABLE_RUNTIMES)
+set(NEED_LIBC_HDRGEN FALSE)
+if("libc" IN_LIST LLVM_ENABLE_RUNTIMES)
+  set(NEED_LIBC_HDRGEN TRUE)
+else()
+  foreach(_name ${LLVM_RUNTIME_TARGETS})
+    if("libc" IN_LIST RUNTIMES_${_name}_LLVM_ENABLE_RUNTIMES)
+      set(NEED_LIBC_HDRGEN TRUE)
+      break()
+    endif()
+  endforeach()
+endif()
+if(NEED_LIBC_HDRGEN)
   # To build the libc runtime, we need to be able to build few libc build
   # tools from the "libc" project. So, we add it to the list of enabled
   # projects.
@@ -177,6 +188,7 @@ if ("libc" IN_LIST LLVM_ENABLE_RUNTIMES)
     list(APPEND LLVM_ENABLE_PROJECTS "libc")
   endif()
 endif()
+unset(NEED_LIBC_HDRGEN)
 
 # LLVM_ENABLE_PROJECTS_USED is `ON` if the user has ever used the
 # `LLVM_ENABLE_PROJECTS` CMake cache variable.  This exists for