lto-plugin: use locking only for selected targets

For now, support locking only for linux targets that are different from
riscv* where the target depends on libatomic (and fails during
bootstrap).

	PR lto/106170

lto-plugin/ChangeLog:

	* configure.ac: Configure HAVE_PTHREAD_LOCKING.
	* lto-plugin.c (LOCK_SECTION): New.
	(UNLOCK_SECTION): New.
	(claim_file_handler): Use the newly added macros.
	(onload): Likewise.
	* config.h.in: Regenerate.
	* configure: Regenerate.