[AVR32] Fix compile error with gcc 4.1

gcc 4.1 doesn't seem to like const variables as inline assembly
outputs. Drop support for reading 64-bit values using get_user() so
that we can use an unsigned long to hold the result regardless of the
actual size. This should be safe since many architectures, including
i386, doesn't support reading 64-bit values with get_user().

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>