linux-user: assume __NR_gettid always exists

The gettid syscall was introduced in Linux 2.4.11. This is old enough
that we can assume it always exists and thus not bother with the
conditional backcompat logic.

Signed-off-by: Daniel P. Berrangé <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Laurent Vivier <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Laurent Vivier <[email protected]>
(cherry picked from commit 184943d827ce09375284e6fbb9fd5eeb9e369529)

Author: berrange

linux-user/syscall.c

@@ -250,15 +250,7 @@ static type name (type1 arg1,type2 arg2,type3 arg3,type4 arg4,type5 arg5,	\
 #define TARGET_NR__llseek TARGET_NR_llseek
 #endif
 
-#ifdef __NR_gettid
 _syscall0(int, gettid)
-#else
-/* This is a replacement for the host gettid() and must return a host
-   errno. */
-static int gettid(void) {
-    return -ENOSYS;
-}
-#endif
 
 /* For the 64-bit guest on 32-bit host case we must emulate
  * getdents using getdents64, because otherwise the host