[libc] Do not install to the default triple in GPU mode

[jhuber6]

Summary

Currently this logic causes the libcgpu.a to be installed to the
user's default triple. The GPU target isn't really true to this in a
cross-compiling sense so we should just default to the regular /lib
directory

[jhuber6]

Side note, I'm going to try to get back into the libc work now that the fork has happened. Hopefully I'll have some actual testing running soon.

libc/lib/CMakeLists.txt

 add_entrypoint_library(
   ${LIBC_TARGET}
   DEPENDS
   ${TARGET_LLVMLIBC_ENTRYPOINTS}
 )
 set_target_properties(
   ${LIBC_TARGET}
   PROPERTIES
     ARCHIVE_OUTPUT_NAME ${LIBC_ARCHIVE_NAME}
 )
 
 if(LIBC_TARGET_TRIPLE)
   set(LIBC_INSTALL_LIBRARY_DIR lib${LLVM_LIBDIR_SUFFIX}/${LIBC_TARGET_TRIPLE})
-elseif(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR)
+elseif(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT LIBC_GPU_BUILD)
   set(LIBC_INSTALL_LIBRARY_DIR
       lib${LLVM_LIBDIR_SUFFIX}/${LLVM_DEFAULT_TARGET_TRIPLE})
 else()
   set(LIBC_INSTALL_LIBRARY_DIR lib${LLVM_LIBDIR_SUFFIX})
 endif()
 
 install(
   TARGETS ${LIBC_TARGET}