ci: fix GCC install in the Travis CI GCC OSX job

ci: fix GCC install in the Travis CI GCC OSX job

A few days ago Travis CI updated their existing OSX images, including
the Homebrew database in the xcode10.1 OSX image that we use.  Since
then installing dependencies in the 'osx-gcc' job fails when it tries
to link gcc@8:

  + brew link gcc@8
  Error: No such keg: /usr/local/Cellar/gcc@8

GCC8 is still installed but not linked to '/usr/local' in the updated
image, as it was before this update, but now we have to link it by
running 'brew link gcc'.  So let's do that then, and fall back to
linking gcc@8 if it doesn't, just to be sure.

Our builds on Azure Pipelines are unaffected by this issue.  The OSX
image over there doesn't contain the gcc@8 package, so we have to
'brew install' it, which already takes care of linking it to
'/usr/local'.  After that the 'brew link gcc' command added by this
patch fails, but the ||-chained fallback 'brew link gcc@8' command
succeeds with an "already linked" warning.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>