1.0.3.16: experimental x86-64/darwin suport

 * fix sb-posix time structs to match headers on darwin
 * comment out mtime sb-posix test as this isn't working ATM
 * add UD2A trap stuff to x86-64 and corresponding word-imm support
 * remove bogus extern-alien-name in boxed_region fixups
 * add compiler parameters for Darwin
 * add x86-64 darwin config file
 * some type safety fixes (proper types) in darwin-dlshim.c
 * use setpgid on Darwin
 * add signal context support x86-64/darwin
 * report trap instead of si_code for trap_Error/trap_Cerror
 * unsigned -> unsigned long in purify.c
 * add mach exception handler support for x86-64/darwin
 * x86-64 assembly hacks to make darwin's assembler happy
 * update x86-64-bsd-os.c to suppot darwin and mach exceptions
 * add x86-64-darwin-os.c/h
 * update LDSO stubs for x86-64 darwin