UM: implement syscall_get_arch()

This patch defines syscall_get_arch() for the um platform.  It adds a
new syscall.h header file to define this.  It copies the HOST_AUDIT_ARCH
definition from ptrace.h.  (that definition will be removed when we
switch audit to use this new syscall_get_arch() function)
Based-on-patch-by: Richard Briggs <rgb@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>
Cc: user-mode-linux-devel@lists.sourceforge.net

arch/x86/um/asm/syscall.h

+#ifndef __UM_ASM_SYSCALL_H
+#define __UM_ASM_SYSCALL_H
+
+#include <uapi/linux/audit.h>
+
+static inline int syscall_get_arch(void)
+{
+#ifdef CONFIG_X86_32
+	return AUDIT_ARCH_I386;
+#else
+	return AUDIT_ARCH_X86_64;
+#endif
+}
+
+#endif /* __UM_ASM_SYSCALL_H */