Fix warning caused by unused-result in bug-atexit3-lib.cc

The test case dlfcn/bug-atexit3-lib.cc calls write and doesn't check the
result.  When building with GCC 6.2, this generates a warning in 'make
check', which is treated as an error.  This patch replaces the call to
write with a call to write_message.

Tested for powerpc64le.