zlib: fix gn error when is_clang=false target_cpu="arm"

Check use_thin_lto instead of thin_lto_enable_optimizations, as the
latter is only defined in a BUILD.gn so can't be checked outside of
it. Plus the intent of https://crrev.com/c/1642730 appears to have
been to error out on LTO in general anyway.

Fixes gn gen --args="is_clang=false target_cpu=\"arm\"" failing with:
ERROR at //third_party/zlib/BUILD.gn:105:17: Undefined identifier
        assert(!thin_lto_enable_optimizations,
                ^----------------------------

Bug: 819294
Change-Id: Ibbcae447b01043edc8a118f4a3a3923f2d0ae261
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1667663


Auto-Submit: Tomasz Śniatowski <tsniatowski@vewd.com>
Commit-Queue: Adenilson Cavalcanti <cavalcantii@chromium.org>
Reviewed-by: Adenilson Cavalcanti <cavalcantii@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#671713}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 28065982319c824cccff912f0ce561f60d054796