linux-user: enable sigaltstack for all architectures

There is no reason to limit sigaltstack syscall to just a few
architectures and pretend it is not implemented for others.

If some architecture is not ready for this, that architecture
should be fixed instead.

This fixes LP#1516408.

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

linux-user/syscall.c

@@ -8292,14 +8292,8 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
         break;
     }
     case TARGET_NR_sigaltstack:
-#if defined(TARGET_I386) || defined(TARGET_ARM) || defined(TARGET_MIPS) || \
-    defined(TARGET_SPARC) || defined(TARGET_PPC) || defined(TARGET_ALPHA) || \
-    defined(TARGET_M68K) || defined(TARGET_S390X) || defined(TARGET_OPENRISC)
         ret = do_sigaltstack(arg1, arg2, get_sp_from_cpustate((CPUArchState *)cpu_env));
         break;
-#else
-        goto unimplemented;
-#endif
 
 #ifdef CONFIG_SENDFILE
     case TARGET_NR_sendfile: