target/alpha: Fix user-only initialization of fpcr

When the representation of fpcr was changed, the user-only
initialization was not updated to match.  Oops.

Fixes: f3d3aad4
Fixes: https://bugs.launchpad.net/bugs/1701835


Reported-by: Bruno Haible <bruno@clisp.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>