Previously, liblink would silently truncate register offset constants
to 32 bits.  For example,

    MOVD $0x200000004(R2),R3

would assemble to

    addis   r31,r2,0
    addi    r3,r31,4

To fix this, limit C_LACON to 32 bit (signed) offsets and introduce a
new C_DACON operand type for larger register offsets.  We don't
implement this currently, but at least liblink will now give an error
if it encounters an address like this.

Change-Id: I8e87def8cc4cc5b75498b0fb543ac7666cf2964e
Reviewed-on: https://go-review.googlesource.com/1758Reviewed-by: Minux Ma <minux@golang.org>