MIPS: Fix FCSR Cause bit handling for correct SIGFPE issue

MIPS: Fix FCSR Cause bit handling for correct SIGFPE issue

Sanitize FCSR Cause bit handling, following a trail of past attempts:

* commit 8c29a9fd4e23 ("MIPS: ptrace: Fix FP context restoration FCSR
regression"),

* commit 636607208bcc ("MIPS: Always clear FCSR cause bits after
emulation"),

* commit 1b97ac60baa3 ("MIPS: Clear [MSA]FPE CSR.Cause after
notify_die()"),

* commit 0d15293986be ("MIPS: Prevent user from setting FCSR cause
bits"),

* commit b54d2901517d ("Properly handle branch delay slots in connection
with signals.").

Specifically do not mask these bits out in ptrace(2) processing and send
a SIGFPE signal instead whenever a matching pair of an FCSR Cause and
Enable bit is seen as execution of an affected context is about to
resume.  Only then clear Cause bits, and even then do not clear any bits
that are set but masked with the respective Enable bits.  Adjust Cause
bit clearing throughout code likewise, except within the FPU emulator
proper where they are set according to IEEE 754 exceptions raised as the
operation emulated executed.  Do so so that any IEEE 754 exceptions
subject to their default handling are recorded like with operations
executed by FPU hardware.

Signed-off-by: Maciej W. Rozycki <macro@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14460/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>