linux-user: correct syscall 123 on sh4

As reported by Cédric VINCENT:

The syscall #123 on SH4 should be "TARGET_NR_cacheflush" instead of
"TARGET_NR_modify_ldt" [1].  The only consequence of this misnaming is
that many "Unsupported syscall" warnings are issued when emulating JIT
compilers.
Reported-by: NCédric VINCENT <cedric.vincent@st.com>
Signed-off-by: NRiku Voipio <riku.voipio@linaro.org>

linux-user/sh4/syscall_nr.h

@@ -125,7 +125,7 @@
 #define TARGET_NR_clone		120
 #define TARGET_NR_setdomainname	121
 #define TARGET_NR_uname		122
-#define TARGET_NR_modify_ldt		123
+#define TARGET_NR_cacheflush		123
 #define TARGET_NR_adjtimex		124
 #define TARGET_NR_mprotect		125
 #define TARGET_NR_sigprocmask	126