Allow larger immediate values in fixnum arithmetic on x86-64.

Allow larger immediate values in fixnum arithmetic on x86-64.

Arithmetic on tagged fixnums currently assembles only constant fixnums
of type (SIGNED-BYTE 29) as immediate arguments to the machine
instructions. When N-FIXNUM-TAG-BITS is less than 3 a larger range of
fixnums could be treated this way. This is desirable as it avoids the
costs of the alternative, namely to put the value into the constant
pool.

So change this type to (SIGNED-BYTE (- 32 N-FIXNUM-TAG-BITS)).

Extend an existing test to cover constants in this range, too.

Many thanks to Paul Khuong for help in finding a name for the type.