cmd/ld: fix bad merge
CL 7504044 accidentally reverted part of CL 7891044 and 7552045, this CL
bring those part back.

R=golang-dev
TBR=rsc
CC=golang-dev
https://codereview.appspot.com/7950045
cmd/ld: don't generate DW_AT_type attr for unsafe.Pointer to match gcc behavior
gcc generates only attr DW_AT_byte_size for DW_TAG_pointer_type of "void *",
but we used to also generate DW_AT_type pointing to imaginary unspecified
type "void", which confuses some gdb.
This change makes old Apple gdb 6.x (specifically, Apple version gdb-1515)
accepts our binary without issue like this:
(gdb) b 'main.main'
Die: DW_TAG_unspecified_type (abbrev = 10, offset = 47079)
    has children: FALSE
    attributes:
        DW_AT_name (DW_FORM_string) string: "void"
Dwarf Error: Cannot find type of die [in module /Users/minux/go/go2.hg/bin/go]

Special thanks to Russ Cox for pointing out the problem in comment #6 of
CL 7891044.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7744051
cmd/ld: portability fixes
fix code that implicitly assumes little-endian machines.

R=golang-dev, bradfitz, rsc, alex.brainman
CC=golang-dev
https://codereview.appspot.com/6792043