cmd/ld: set ELF ABI version for ppc64x

On ppc64, there are three ELF ABI versions an ELF file can request.
Previously, we used 0, which means "unspecified".  On our test
machines, this meant to use the default (v1 for big endian and v2 for
little endian), but apparently some systems can pick the wrong ABI if
neither is requested.  Leaving this as 0 also confuses libbfd, which
confuses gdb, objdump, etc.

Fix these problems by specifying ABI v1 for big endian and v2 for
little endian.

Change-Id: I4d3d5478f37f11baab3681a07daff3da55802322
Reviewed-on: https://go-review.googlesource.com/1800
Reviewed-by: Minux Ma <minux@golang.org>
liblink: fail for too-large register offset constants

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/1758
Reviewed-by: Minux Ma <minux@golang.org>