cleaning up syscalls in preparation for x86_64 port

- hide all the legacy xxxxxx32 name cruft in syscall.h so the actual
source files can be clean and uniform across all archs.

- cleanup llseek/lseek and mmap2/mmap handling for 32/64 bit systems

- alternate implementation for nice if the target lacks nice syscall

src/linux/setgroups.c

@@ -1,9 +1,7 @@
 #include <unistd.h>
-#define SYSCALL_STANDALONE
 #include "syscall.h"
 
 int setgroups(int count, const gid_t list[])
 {
-	/* this depends on our gid_t being 32bit */
-	return syscall2(__NR_setgroups32, count, (long)list);
+	return syscall2(__NR_setgroups, count, (long)list);
 }