target/xtensa: add linux-user support

Import list of syscalls from the kernel source. Conditionalize code/data
that is only used with softmmu. Implement exception handlers. Implement
signal hander (only the core registers for now, no coprocessors or TIE).

Cc: Riku Voipio <riku.voipio@iki.fi>
Cc: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

linux-user/syscall.c

@@ -709,6 +709,8 @@ static inline int regpairs_aligned(void *cpu_env, int num)
         return 0;
     }
 }
+#elif defined(TARGET_XTENSA)
+static inline int regpairs_aligned(void *cpu_env, int num) { return 1; }
 #else
 static inline int regpairs_aligned(void *cpu_env, int num) { return 0; }
 #endif