Eliminate redundant underscore symbols in sed arguments in Makefile
(revert commit 75b9e67 partially)

* src/Makefile.am (atomic_ops/generalize-small.h,
atomic_ops/generalize-arithm.h,
atomic_ops/sysdeps/loadstore/char_acquire_release_volatile.h,
atomic_ops/sysdeps/loadstore/int_acquire_release_volatile.h,
atomic_ops/sysdeps/loadstore/short_acquire_release_volatile.h,
atomic_ops/sysdeps/loadstore/char_atomic_load_store.h,
atomic_ops/sysdeps/loadstore/int_atomic_load_store.h,
atomic_ops/sysdeps/loadstore/short_atomic_load_store.h): Remove
redundant '_' (underscore) symbol in sed "search" and "replace-to"
strings.