libstdc++: Default to mutex-based atomics on RISC-V

libstdc++: Default to mutex-based atomics on RISC-V

The RISC-V port requires libatomic to be linked in order to resolve
various atomic functions, which results in builds that have
"--with-libstdcxx-lock-policy=auto" defaulting to mutex-based locks.
Changing this to direct atomics breaks the ABI, this forces the auto
detection mutex-based atomics on RISC-V in order to avoid a silent ABI
break for users.

See Bug 84568 for more discussion.  In the long run there may be a way
to get the higher-performance atomics without an ABI flag day, but
that's going to be a much more complicated operation.  We don't even
have support for the inline atomics yet, but given that some folks have
been discussing hacks to make these libatomic routines appear implicitly
it seems prudent to just turn off the automatic detection for RISC-V.

libstdc++-v3/ChangeLog:

	* acinclude.m4 (GLIBCXX_ENABLE_LOCK_POLICY): Force auto to mutex
	for RISC-V.
	* configure: Regenerate.