[gold][patch] Fix non-PIC warning to print only when building position-independent output

[Cary Coutant]

>>       * powerpc.cc (Target_powerpc::check_non_pic): Assert that output is
>>       position independent.
>>       * sparc.cc (Target_sparc::check_non_pic): Likewise.
>>       * x86_64.cc (Target_x86_64::check_non_pic): Likewise.
>
> This is OK.

Committed. I also fixed a typo in a previous ChangeLog entry
(s/readsymc.cc/readsyms.cc/).

-cary