target-i386: fix PSE36 mode

(pde & 0x1fe000) is a 32-bit integer; when shifting it
into bits 39-32 the result is zero.  Fix it by making the
mask (and thus the result of the AND) a 64-bit integer.

Reported by Coverity.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>