Save and restore FPU registers around signal handlers.
Fixes the random crashes in sh(1).

ok guenther@

SROP mitigation. sendsig() stores a (per-process ^ &sigcontext) cookie
inside the sigcontext. sigreturn(2) checks syscall entry was from the
exact PC addr in the (per-process ASLR) sigtramp, verifies the cookie,
and clears it to prevent sigcontext reuse.
not yet tested on landisk, sparc, *88k, socppc.
ok kettenis

Determine whether we're currently on the alternative signal stack
dynamically, by comparing the stack pointer against the altstack
base and size, so that you get the correct answer if you longjmp
out of the signal handler, as tested by regress/sys/kern/stackjmp/.
Also, fix alt stack handling on vax, where it was completely broken.

Testing and corrections by miod@, krw@, tobiasu@, pirofti@

Late spring cleaning of the arm code for old dusty bits we do not want to
keep:
- remove bootconfig parameter passing feature (unused).
- unifdef __PROG32 and remove all remains of arm26 code.
- remove ARMFPE support (unused).
- remove support for ARM2, ARM2AS, ARM3, ARM6, ARM7, ARM7TDMI and StrongARM
processor families, and the related silicon bug workarounds (especially
the SA-110 STM^ bug).
- remove cpu_functions no longer necessary after previous removals.
- remove ARM32_DISABLE_ALIGNMENT_FAULTS option (unused).
- make FIQ support conditional on option FIQ (unused, but may be eventually).

Discussed with drahn@ and jasper@ long ago, I was sitting on this commit for
no good reason.

Normalize sentinel. Use _MACHINE_*_H_ and _<ARCH>_*_H_ properly and consitently.

Discussed and okay drahn@. Okay deraadt@.

Remove unused defines.

Expose struct sigcontext, struct sigstat and struct sigaltstack for XPG4.2

Don't include cdefs.h if _LOCORE is defined and hide some more defines
unless __BSD_VISIBLE or _LOCORE are set. OK deraadt@

convert _FOO_SOURCE -> __FOO_VISIBLE in machine. OK deraadt@

branches: 1.1.2;
Arm port, NetBSD codebase stripped down, 32bit only support.

SROP mitigation. sendsig() stores a (per-process ^ &sigcontext) cookie
inside the sigcontext. sigreturn(2) checks syscall entry was from the
exact PC addr in the (per-process ASLR) sigtramp, verifies the cookie,
and clears it to prevent sigcontext reuse.
not yet tested on landisk, sparc, *88k, socppc.
ok kettenis

Determine whether we're currently on the alternative signal stack
dynamically, by comparing the stack pointer against the altstack
base and size, so that you get the correct answer if you longjmp
out of the signal handler, as tested by regress/sys/kern/stackjmp/.
Also, fix alt stack handling on vax, where it was completely broken.

Testing and corrections by miod@, krw@, tobiasu@, pirofti@

Late spring cleaning of the arm code for old dusty bits we do not want to
keep:
- remove bootconfig parameter passing feature (unused).
- unifdef __PROG32 and remove all remains of arm26 code.
- remove ARMFPE support (unused).
- remove support for ARM2, ARM2AS, ARM3, ARM6, ARM7, ARM7TDMI and StrongARM
processor families, and the related silicon bug workarounds (especially
the SA-110 STM^ bug).
- remove cpu_functions no longer necessary after previous removals.
- remove ARM32_DISABLE_ALIGNMENT_FAULTS option (unused).
- make FIQ support conditional on option FIQ (unused, but may be eventually).

Discussed with drahn@ and jasper@ long ago, I was sitting on this commit for
no good reason.

Normalize sentinel. Use _MACHINE_*_H_ and _<ARCH>_*_H_ properly and consitently.

Discussed and okay drahn@. Okay deraadt@.

Remove unused defines.

Expose struct sigcontext, struct sigstat and struct sigaltstack for XPG4.2

Don't include cdefs.h if _LOCORE is defined and hide some more defines
unless __BSD_VISIBLE or _LOCORE are set. OK deraadt@

convert _FOO_SOURCE -> __FOO_VISIBLE in machine. OK deraadt@

branches: 1.1.2;
Arm port, NetBSD codebase stripped down, 32bit only support.