Track undefined sym in the symtab. Remove hack for missing relocs against undef.

Preliminary linking tests, seems to show that the linker
and objcopy are happy to use 'em on spec2k, and the
result runs! (Had to be careful to clobber the old .s
and .o files to make it's testing the right copy).

Haven't tried crosstests yet.

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/889613004
9 files changed