[Bug testsuite/101002] Some powerpc tests fail with -mlong-double-64

["bergner at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101002

--- Comment #7 from Peter Bergner <bergner at gcc dot gnu.org> ---
(In reply to Peter Bergner from comment #6)
> I'm going to test the following to see whether anything bad falls out:
> 
> --- a/gcc/config/rs6000/rs6000-builtin.cc
> +++ b/gcc/config/rs6000/rs6000-builtin.cc
> @@ -710,9 +710,9 @@ rs6000_init_builtins (void)
>       For IEEE 128-bit floating point, always create the type __ieee128.  If
> the
>       user used -mfloat128, rs6000-c.cc will create a define from __float128
> to
>       __ieee128.  */
> -  if (TARGET_LONG_DOUBLE_128 && (!TARGET_IEEEQUAD || TARGET_FLOAT128_TYPE))
> +  if (!TARGET_IEEEQUAD || TARGET_FLOAT128_TYPE)
>      {
> -      if (!TARGET_IEEEQUAD)
> +      if (TARGET_LONG_DOUBLE_128 && !TARGET_IEEEQUAD)
>         ibm128_float_type_node = long_double_type_node;
>        else
>         {

So this passes (powerpc64le-linux) bootstrap and regtesting with no regressions
using a --with-long-double-128 build.  Using a --without-long-double-128 build
(ie, long double == double), signbit-1.c now ICEs with:

signbit-1.c: In function ‘do_signbit_if’:
signbit-1.c:8:64: error: unrecognizable insn:
    8 | int do_signbit_if (__ibm128 a) { return __builtin_signbit (a); }
      |                                                                ^
(insn 6 3 7 2 (set (reg:DF 120)
        (float_truncate:DF (reg/v:IF 119 [ a ]))) "signbit-1.c":8:41 -1
     (nil))
during RTL pass: vregs
signbit-1.c:8:64: internal compiler error: in extract_insn, at recog.cc:2791

...so it seems we do have some inherent assumptions baked into the backend. :-(
There are a few other testsuite regressions due to a linker warning the
testsuite wouldn't ignore:

/opt/binutils-power10/bin/ld: /tmp/ccysrhL7.o uses 64-bit long double,
/lib64/libc.so.6 uses 128-bit long double