sh3: siglongjmp - restore register context first (PR lib/57946)

tests/lib/libc/setjmp/t_sigstack: Add missing comment for 1.10.

PR lib/57946

arm has been fixed

i386 longjmp: Restore stack first, then signal mask.

Otherwise, a pending signal may be delivered on the wrong stack when
we restore the signal mask.

While here:

- Tidy the code a little bit.
- Sp

i386 longjmp: Restore stack first, then signal mask.

Otherwise, a pending signal may be delivered on the wrong stack when
we restore the signal mask.

While here:

- Tidy the code a little bit.
- Sprinkle comments to explain what's going on.
- Use forward branches for statically predicted not-taken.
=> val==0 is unlikely in longjmp

PR lib/57946

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amd64 longjmp: Restore stack first, then signal mask.

Otherwise, a pending signal may be delivered on the wrong stack when
we restore the signal mask.

While here:

- Tidy the code a little bit.
- S

amd64 longjmp: Restore stack first, then signal mask.

Otherwise, a pending signal may be delivered on the wrong stack when
we restore the signal mask.

While here:

- Tidy the code a little bit.
- Sprinkle comments to explain what's going on.
- Use `xorl %eXX,%eXX' instead of `xorq %rXX,%rXX'.
=> Same effect, one byte shorter, breaks dep chain on more uarches.
- Use forward branches for statically predicted not-taken.
=> val==0 is unlikely in longjmp

PR lib/57946

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longjmp(3) t_sigstack: Fix fencepost error.

Extremely unlikely to cause trouble, but let's just turn that into
`never' to keep it easier for readers.

PR lib/57946

longjmp(3) t_sigstack: Use a sigaltstack per handler entry.

longjmp evidently doesn't reset the state of whether the process is
executing on the alternate signal stack. So when we re-enter the
sign

longjmp(3) t_sigstack: Use a sigaltstack per handler entry.

longjmp evidently doesn't reset the state of whether the process is
executing on the alternate signal stack. So when we re-enter the
signal handler, the alternate stack appears to be still in use, and
the system chooses the original stack for the second call to the
signal handler -- which trips our assertion asking to verify that the
signal handler is always using an alternate stack.

Not strictly necessary for the signal handler to use an alternate
stack on re-entry, but this makes it clearer that the signal handler
itself is always using the alternate stack so we can verify that the
interrupted code is _not_ in the signal handler.

With this change, the test now passes on aarch64.

PR lib/57946

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longjmp(3) t_sigstack: Note aarch64 seems to DTRT.

But only by code inspection; it appears to have another problem: on
re-entry, the signal handler is called on the normal stack, not on
the alternat

longjmp(3) t_sigstack: Note aarch64 seems to DTRT.

But only by code inspection; it appears to have another problem: on
re-entry, the signal handler is called on the normal stack, not on
the alternate signal stack.

PR lib/57946

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longjmp(3) t_sigstack: Print which entry failed.

PR lib/57946

longjmp(3): Test signal mask vs stack restore with siglongjmp too.

PR lib/57946

longjmp(3): Paranoia: more error checking in PR lib/57946 test.

longjmp(3): Add test for PR lib/57946.