Create a custom config for LLVM libc

Since LLVM libc lacks an AArch64 config, LIBC_CONFIG_PATH is set to
the arm subdirectory to force it to use the existing AArch32 config.
However, this has the side effect of never loading the generic
baremetal config.json file, which is instead in the parent directory.

In order to combine the entrypoints in the arm config with the
values set in the generic baremetal config, this patch copies both
to the same location to be used in the build.

CMakeLists.txt

@@ -392,6 +392,19 @@ if(LLVM_TOOLCHAIN_C_LIBRARY STREQUAL llvmlibc)
     # find one for every libc type. We have no current setup to run the LLVM
     # libc test suite.
     add_custom_target(check-llvmlibc)
+
+    # LLVM libc lacks a configuration for AArch64, but the AArch32 one works
+    # fine. However, setting the configuration for both architectures to the
+    # arm config directory means the baremetal config.json is never loaded,
+    # as it resides in the directory above. To ensure both are used, copy
+    # them to the same location and point libc to that.
+    set(LIBC_CFG_DIR ${CMAKE_BINARY_DIR}/llvmlibc-config)
+    file(COPY
+        ${llvmproject_SOURCE_DIR}/libc/config/baremetal/config.json
+        ${llvmproject_SOURCE_DIR}/libc/config/baremetal/arm/.
+        DESTINATION
+        ${LIBC_CFG_DIR}
+    )
 endif()
 
 add_subdirectory(
@@ -911,7 +924,7 @@ function(
         ${common_cmake_args}
         -DLIBC_TARGET_TRIPLE=${target_triple}
         -DLIBC_HDRGEN_EXE=${LIBC_HDRGEN}
-        -DLIBC_CONFIG_PATH=${llvmproject_SOURCE_DIR}/libc/config/baremetal/arm # llvmlibc has no AArch64 bare-metal configuration, but the AArch32 one is fine
+        -DLIBC_CONFIG_PATH=${LIBC_CFG_DIR}
         -DLIBC_CONF_TIME_64BIT=ON
         -DLLVM_CMAKE_DIR=${LLVM_BINARY_DIR}/lib/cmake/llvm
         -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON