1.0.38.12: Fix FP traps on PPC/Linux.

1.0.38.12: Fix FP traps on PPC/Linux.

  * Linux on most platforms, including PPC, kicks off its signal
handlers with a cleared FP control word.  We already have a hook to deal
with this, so enable it.

  * The implementation of said hook on PPC/Linux was broken, largely due
to a variable-size mismatch in a KLUDGE it uses.  Fixed and documented
the KLUDGE, added support for preserving the current rounding mode, and
enabled the actual restoration of the FP control word.

  * NetBSD isn't the only target which requires :INVALID exceptions to
be disabled, it also matters on PPC.  Fixed the default control mode.

  * Fix up the test suite to reflect the current expectations for
float.pure.lisp tests.