Improve signals and memory protection

- Document sigaction()
- Simplify New Technology fork() code
- Testing and many bug fixes for mprotect()
- Distribute Intel Xed ILD in the amalgamation
- Turn Xed enums into defines to avoid DWARF bloat
- Improve polyfilling of SA_SIGINFO on BSDs and fix bugs
- setpgid(getpid(), getpid()) on Windows will ignore CTRL-C
- Work around issues relating to NT mappings being executable
- Permit automatic executable stack override via `ape_stack_pf`

libc/runtime/jmpstack.S

@@ -31,7 +31,7 @@ _jmpstack:
 	mov	%rcx,%rsi
 	mov	%r8,%rdx
 	mov	%r9,%rcx
-	xor	%rbp,%rbp
+	xor	%ebp,%ebp
 	call	*%rax
-	ud2
+	.unreachable
 	.endfn	_jmpstack,globl,hidden