PowerPC: PR libgcc/97543, fix 64-bit long double issues

PowerPC: PR libgcc/97543, fix 64-bit long double issues

There are two issues in PR libgcc/97543 which shows up if you build a GCC
compiler with long double defaulting to 64-bit instead of 128-bit with IBM
extended double:

    1)	The first issue was the t-linux file forced the entire libgcc library
	to be compiled with the -mlong-double-128 option.

    2)	The second issue is that the GNU attribute #4 is set to reflect using
	128-bit long doubles, and you get linker warnings when you use use the
	compiler, since libgcc_s.so indicates 128-bit IBM long doubles were
	used.  I ran into a similar issue with my patches to extend libgcc to
	work if long doubles were configured to use the 128-bit IEEE format
	instead of the 128-bit IBM format.

One feature of the current GNU attribute implementation is if you have a shared
library (such as libgcc_s.so), the GNU attributes for the shared library is an
inclusive OR of all of the modules within the library.  This means if any
module uses the -mlong-double-128 option and uses long double, the GNU
attributes for the library will indicate that it uses 128-bit IBM long
doubles.  If you have a static library, you will get the warning only if you
actually reference a module with the attribute set.

In addition to the issues for 64-bit long doubles, there are similar issues if
long double is configured to be IEEE 128-bit (PR libgcc/97643).

This patch does two things:

    1)	All of the modules that support IBM 128-bit long doubles explicitly set
	the ABI to IBM extended double.

    2) I turned off GNU attributes for building the shared library.

I have tested this by building a compiler on a little endian power9 system
running Linux with long double defaulting to 64-bits using the configure
option: --without-long-double-128, and I verified that the warning no longer is
generated by the linker.

I then built a bootstrap compiler, by first building a non-bootstrap version.
With that non-bootstrap compiler, I built versions of the MPC and MPFR.  Using
those libraries, and the non-bootstrap compiler as the host compiler, I was
able to do a full bootstrap compiler.

There are differences in the regression test suite where the test implicitly
assumed long double was 128-bits or was a float128 test.

Can I install this patch into the master branch?  I would also like to install
it in the GCC 10 branch after an appropriate period.

libgcc/
2020-11-10  Michael Meissner  <meissner@linux.ibm.com>

	PR libgcc/97543
	PR libgcc/97643
	* config/rs6000/t-linux (IBM128_STATIC_OBJS): New make variable.
	Set all objects to use the explicit IBM format.
	(IBM128_SHARD_CFLAGS): New make variable. Set all objects to use
	the explicit IBM format.
	(IBM128_SHARED_CFLAGS): New make variable.
	(IBM128_STATIC_CFLAGS): New make variable.
	(gcc_s_compile): Add -mno-gnu-attribute to all shared library
	modules.