Move SC_* defines into <machine/setjmp.h>, and rename them,
so that <machine/signal.h> is not needed in setjmp.S.

Suggested by kettenis@ long ago, OK deraadt@

Use __ASSEMBLER__ instead of __LANGUAGE_ASSEMBLY.
The latter is not defined by clang.

OK guenther@, kettenis@

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@

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

Discussed and okay drahn@. Okay deraadt@.

Use protected version of register_t in struct sigcontext for the
case where _XOPEN_SOURCE was defined but BSD-specific types are
not visible.

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

convert _FOO_SOURCE -> __FOO_VISIBLE in machine. OK deraadt@

backout miod's change because he did not compile libc.

Remove older-than-gramps poor man's assym.h defines; .S files do not even need
these constants.

Remove advertising clause from UCB licenses; ok deraad@

spacing

initial mips64