linux-user: Add AArch64 support

linux-user: Add AArch64 support

This patch adds support for AArch64 in all the small corners of
linux-user (primarily in image loading and startup code).

Signed-off-by: Alexander Graf <[email protected]>
Signed-off-by: John Rigby <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>
Message-id: 1378235544[email protected]
Message-id: 1368505980[email protected]
[PMM:
 * removed some unnecessary #defines from syscall.h
 * catch attempts to use a 32 bit only cpu with aarch64-linux-user
 * termios stuff moved into its own patch
 * we specify our minimum uname version here now
]
Signed-off-by: Peter Maydell <[email protected]>