Revert of mesa: Disable support for loading libtxc_dxtn.so if msan is enabled.

Revert of mesa: Disable support for loading libtxc_dxtn.so if msan is enabled. (patchset #2 id:20001 of https://codereview.chromium.org/2876693003/ )

Reason for revert:
Large number of perf regressions on mac.

BUG=722473
Original issue's description:
> mesa: Disable support for loading libtxc_dxtn.so if msan is enabled.
> 
> The libtxc_dxtn.so library supports loading textures in S3TC
> format. This change disables code in mesa that loads libtxc_dxtn.so
> if msan is enabled, as the library will be loaded from a system
> directory, which may cause spurious msan reports because msan expects
> most libraries in the process to be compiled with msan.
> 
> For some reason, msan only appears to break when compiling with -Os,
> as revealed by a recent attempt [0] to enable -Os on Linux and Mac.
> 
> [0] https://codereview.chromium.org/2864383003/
> 
> BUG=660216
> R=senorblanco@chromium.org,thakis@chromium.org
> 
> Review-Url: https://codereview.chromium.org/2876693003
> Cr-Commit-Position: refs/heads/master@{#471045}
> Committed: https://chromium.googlesource.com/chromium/src/+/97b99808511f8918d00add266a8555ba5e21813c

TBR=senorblanco@chromium.org,thakis@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=660216

[pcc1, 2017-05-15 19:56:09]

The CQ bit was checked by pcc@chromium.org

[pcc1, 2017-05-15 19:56:10]

Created Revert of mesa: Disable support for loading libtxc_dxtn.so if msan is
enabled.

[pcc1, 2017-05-15 19:56:26]

The CQ bit was unchecked by pcc@chromium.org

[pcc1, 2017-05-15 19:56:55]

Oops, created a revert of the wrong CL, closing.