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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}