XL: Fix the use of the -qhalt flag for varying OSs

The valid settings to pass to the IBM XL compiler for the -qhalt flag
vary widely by language and platform.  Based on existing documentation,
the following table shows which error levels are valid to pass to
-qhalt= since -qhalt=e is not always available.

OS       |  xlc  |  xlC  |   xlf   |
---------|-------|------------------
AIX      | iwes  | iw s  | ilwesu  |
BlueGene | iwes  | iwes  | ilwesu  |
OS X     | iwesu | iwesu | ilwesu  |
Linux    |  w    |  w    | ilwesu  |
1 job for xl-add-c++11-flags
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