changes in sbcl-2.2.3 relative to sbcl-2.2.2:
* minor incompatible change: SB-THREAD:MUTEX-OWNER may return :THREAD-DEAD
if the apparent owner either exited nearly instantly after releasing the
mutex (and is not now the owner), or died and never released it.
* minor incompatible change: building the system with the simple semi-space
copying collector is no longer supported.
* minor incompatible change: support for PPC/Darwin has been removed.
* platform support:
** fix regressions in threads on RISC-V. (lp#1962598)
** threads are now enabled by default on RISC-V.
** The generational garbage collector is now supported on MIPS.
* optimization: fasls containing standard object literals are now smaller
and load more efficiently.
* optimization: faster arithmetic (*-+) on word-sized integers when the
result is not known to fit into a word (ARM64 and x86-64).
* bug fix: EQness of constants is now always preserved when block compiling.