rs6000: Fix up *intrin.h for C89 [PR104239]

rs6000: Fix up *intrin.h for C89 [PR104239]

When writing testcases for the previously posted patch, I have noticed
that 3 of the headers aren't valid C89 (I didn't have any dg-options
so -ansi -pedantic-errors was implied and these errors were reported).

The following patch fixes those, ok for trunk?

Note, as can be seen even in this patch, seems older rs6000/*intrin.h
headers uglify not just argument names (__A instead of A etc.), but also
automatic variable names and other local identifiers, while e.g. emmintrin.h
or bmi2intrin.h clearly uglify only the argument names and not local
variables.  I think that should be fixed but don't have time for that myself
(libstdc++ or e.g. the x86 headers uglify everything; this is so that one
can
 #define result a + b
 #include <x86intrin.h>
etc.).

2022-01-26  Jakub Jelinek  <jakub@redhat.com>

	PR target/104239
	* config/rs6000/emmintrin.h (_mm_sad_epu8): Use __asm__ instead of
	asm.
	* config/rs6000/smmintrin.h (_mm_minpos_epu16): Declare iterator
	before for loop instead of for init clause.
	* config/rs6000/bmi2intrin.h (_pext_u64): Likewise.

	* gcc.target/powerpc/pr104239-3.c: New test.