PPC: [compiler] Remove broken support for undetectable strings.

PPC: [compiler] Remove broken support for undetectable strings.

Port b62dbf1efdec68ae709b0e91d3b7c13171b720c9

Original commit message:
    Support for undetectable strings was officially dropped in
    https://codereview.chromium.org/916753002, but the compilers
    weren't fixed properly.

R=bmeurer@chromium.org, jyan@ca.ibm.com, dstence@us.ibm.com, joransiu@ca.ibm.com
BUG=

Committed: https://crrev.com/750b7f7a960ae129221d284e8454fb7860b31d25
Cr-Commit-Position: refs/heads/master@{#30390}

[john.yan, 2015-08-26 19:10:50]

lgtm

[dstence, 2015-08-26 19:22:03]

lgtm

[MTBrandyberry, 2015-08-26 19:22:42]

The CQ bit was checked by mbrandy@us.ibm.com

[commit-bot: I haz the power, 2015-08-26 19:22:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1312473012/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1312473012/1

[commit-bot: I haz the power, 2015-08-26 19:45:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-08-26 19:45:56]

Patchset 1 (id:??) landed as
https://crrev.com/750b7f7a960ae129221d284e8454fb7860b31d25
Cr-Commit-Position: refs/heads/master@{#30390}