Fix native_exec hotp assertion

My last change accidentally flipped the logic of the assert, as well as
leaving behind an undeclared variable error for real_retaddr.

Review URL: https://codereview.appspot.com/7375056

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@1949 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6
i#786: Support linking with GNU gold

Call gcc -Wl,--version and look for "GNU gold".  If we find it, we can
use command line options to get our preferred base and special symbols
instead of the linker script.

We have to keep the linker script support around for dynamorio_so_start
and older GNU ld versions.

Gold currently warns on our private string routine replacements in
unit_tests, but links successfully.

Fixes issue 786.

Review URL: https://codereview.appspot.com/7372052

git-svn-id: https://dynamorio.googlecode.com/svn/trunk@1950 49cc7528-f6fd-11dd-9d1a-b59b2e1864b6