rs6000: __Uglify non-uglified local variables in headers

rs6000: __Uglify non-uglified local variables in headers

Properly prefix (with "__")  all local variables in shipped headers for x86
compatibility intrinsics implementations.  This avoids possible problems with
usages like:
```
```

2022-02-16  Paul A. Clarke  <pc@us.ibm.com>

gcc
	PR target/104257
	* config/rs6000/bmi2intrin.h: Uglify local variables.
	* config/rs6000/emmintrin.h: Likewise.
	* config/rs6000/mm_malloc.h: Likewise.
	* config/rs6000/mmintrin.h: Likewise.
	* config/rs6000/pmmintrin.h: Likewise.
	* config/rs6000/smmintrin.h: Likewise.
	* config/rs6000/tmmintrin.h: Likewise.
	* config/rs6000/xmmintrin.h: Likewise.