Rename __{float,double}_u to __x86_{float,double}_u to avoid pulluting the namespace.

Rename __{float,double}_u to __x86_{float,double}_u to avoid pulluting the namespace.

I have a build failure on NetBSD as the namespace pollution avoidance causes
a direct hit with the system /usr/include/math.h
=======================================================================

In file included from /usr/src/local/gcc/obj/gcc/include/emmintrin.h:31,
                 from /usr/src/local/gcc/obj/x86_64-unknown-netbsd10.99/libstdc++-v3/include/ext/random:45,
                 from /usr/src/local/gcc/libstdc++-v3/include/precompiled/extc++.h:65:
/usr/src/local/gcc/obj/gcc/include/xmmintrin.h:75:15: error: conflicting declaration 'typedef float __float_u'
   75 | typedef float __float_u __attribute__ ((__may_alias__, __aligned__ (1)));
      |               ^~~~~~~~~
In file included from /usr/src/local/gcc/obj/x86_64-unknown-netbsd10.99/libstdc++-v3/include/cmath:47,
                 from /usr/src/local/gcc/obj/x86_64-unknown-netbsd10.99/libstdc++-v3/include/x86_64-unknown-netbsd10.99/bits/stdc++.h:114,
                 from /usr/src/local/gcc/libstdc++-v3/include/precompiled/extc++.h:32:
/usr/src/local/gcc/obj/gcc/include-fixed/math.h:49:7: note: previous declaration as 'union __float_u'
   49 | union __float_u {

gcc/ChangeLog:

	PR target/115796
	* config/i386/emmintrin.h (__float_u): Rename to ..
	(__x86_float_u): .. this.
	(_mm_load_sd): Ditto.
	(_mm_store_sd): Ditto.
	(_mm_loadh_pd): Ditto.
	(_mm_loadl_pd): Ditto.
	* config/i386/xmmintrin.h (__double_u): Rename to ..
	(__x86_double_u): .. this.
	(_mm_load_ss): Ditto.
	(_mm_store_ss): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/pr115796.c: New test.