Add handeling of R_X86_64_GOTPCRELX and R_X86_64_REX_GOTPCRELX.
They are not that old, so I provided ifdef-guards for them.
According to x86-64 ABI specification paper, they should be
generated instead of R_X86_64_GOTPCREL for cases when relaxation
is possible.

At this moment we can handle them the same way like R_X86_64_GOTPCREL.

[0] https://github.com/hjl-tools/x86-psABI/wiki/x86-64-psABI-r249.pdf

Fixes: #397

Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Reported-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>