Generate absl .def file at build time.

Instead of generating absl .def files at roll time, this CL switches
the build config to generate the required .def file on the fly while
building.

The advantages are:

1. No need to re-generate .def files when something external to absl
changes and affects .def files (e.g. when a new compiler optimization
it enabled).
2. No need to hard-code supported build flavors in the .def generator
script, resulting in multiple .def files checked-in.

On the other hand, this makes the build slightly slower than it
is today because reading all absl object files to generate the
.def file can take a few seconds.

Bug: 1046390
Change-Id: If3a8fb3dca0ed75dbd85655cff1263853dcc146a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2270094


Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824844}