remove standalone syscall cruft

this was originally written for an early draft of the library where
non-standard functions would reside in a static library separate from
the shared libc.so, which would implement a pure standard. the idea
was not to depend on an implementation-dependent __syscall_ret
function in the main libc. but it turned out to be better to put
everything in a single library for both static and dynamic linking
uses, and thus the (incomplete) remnants of this feature were just
enlarging the source and binary.

src/linux/mount.c

@@ -1,5 +1,4 @@
 #include <sys/mount.h>
-#define SYSCALL_STANDALONE
 #include "syscall.h"
 
 int mount(const char *special, const char *dir, const char *fstype, unsigned long flags, const void *data)