[CMake] Split -gx strip flag into -g -x
[CMake] Split -gx strip flag into -g -x

llvm-strip doesn't handle -gx spelling, so we need to split these
as two separate flags.

Differential Revision: https://reviews.llvm.org/D50684

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@339639 91177308-0d34-0410-b5e6-96231b3b80d8

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