fix OpenBSD linkage (wxneeded)

There is two types of options passed directly to 'ld'
(and not to 'gcc' driver):
    - CONF_LD_LINKER_OPTS_STAGE$4
    - EXTRA_LD_OPTS

This changedoes two things:

- split 'EXTRA_LD_OPTS' into two variables:
    - EXTRA_LD_OPTS (accepts 'gcc' wrapper options)
    - EXTRA_LD_LINKER_OPTS (accepts raw 'ld' options)

- wraps all LD_LINKER options as '-Wl,' when passed
  to 'gcc' driver.

Fixes https://phabricator.haskell.org/D2776



Signed-off-by: Sergei Trofimovich <siarheit@google.com>