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/xtensa/target_signal.h

@@ -0,0 +1,28 @@
+#ifndef XTENSA_TARGET_SIGNAL_H
+#define XTENSA_TARGET_SIGNAL_H
+
+#include "cpu.h"
+
+/* this struct defines a stack used during syscall handling */
+
+typedef struct target_sigaltstack {
+    abi_ulong ss_sp;
+    abi_int ss_flags;
+    abi_ulong ss_size;
+} target_stack_t;
+
+/*
+ * sigaltstack controls
+ */
+#define TARGET_SS_ONSTACK     1
+#define TARGET_SS_DISABLE     2
+
+#define TARGET_MINSIGSTKSZ    2048
+#define TARGET_SIGSTKSZ       8192
+
+static inline abi_ulong get_sp_from_cpustate(CPUXtensaState *state)
+{
+    return state->regs[1];
+}
+
+#endif