[testsuite] gcc.dg/vect/pr48765: fix option conflict

[Janis Johnson <janis_johnson at mentor dot com>]

Test gcc.dg/vect/pr48765, run only for powerpc*-*-*, unconditionally
adds -mcpu=power6 to the command line.  It also adds -m64, although
tests should rely on different multilib flags for that and skip the
test if not lp64 if the test can only be run with -m64.

For all of the Power multilib flags that I normally test, the vect test
infrastructure either skips the tests (for E500) or adds "-maltivec
-mcpu=970" which conflicts with the "-mcpu=power6" in this test and
causes the test to fail to compile.

The "-m64" definitely has to go.  If the "-mcpu=power6" needs to stay in
order to use the flags that triggered the ICE for which the test was
added, then the test should be skipped if some other -mcpu option will
override it.  Another option is to remove that and figure that
occasionally someone will run tests for power6.

This version keeps "-mcpu=power6" and skips the tests if another -mcpu
is already specified.  OK for trunk, and for 4.7 when the branch opens?

Janis

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