The change in major version (and the increment from Darwin19 to 20)
caused libtool tests to fail which resulted in incorrect build settings
for shared libraries.

We take this opportunity to sort out the shared undefined symbols state
rather than propagating the current unsound behaviour into a new rev.

This change means that we default to the case that missing symbols are
considered an error, and if one wants to allow this intentionally, the
confiuration for that case should be set appropriately.

Three existing cases need undefined dynamic lookup:
 libitm, where there is already a configuration mechanism to add the
         flags.
 libcc1, where we add simple configuration to add the flags for Darwin.
 libsanitizer, where we can add to the existing extra flags.

libcc1/ChangeLog:

	PR target/97865
	* Makefile.am: Add dynamic_lookup to LD flags for Darwin.
	* configure.ac: Test for Darwin host and set a flag.
	* Makefile.in: Regenerate.
	* configure: Regenerate.

libitm/ChangeLog:

	PR target/97865
	* configure.tgt: Add dynamic_lookup to XLDFLAGS for Darwin.
	* configure: Regenerate.

libsanitizer/ChangeLog:

	PR target/97865
	* configure.tgt: Add dynamic_lookup to EXTRA_CXXFLAGS for
	Darwin.
	* configure: Regenerate.

ChangeLog:

	PR target/97865
	* libtool.m4: Update handling of Darwin platform link flags
	for Darwin20.

gcc/ChangeLog:

	PR target/97865
	* configure: Regenerate.

libatomic/ChangeLog:

	PR target/97865
	* configure: Regenerate.

libbacktrace/ChangeLog:

	PR target/97865
	* configure: Regenerate.

libffi/ChangeLog:

	PR target/97865
	* configure: Regenerate.

libgfortran/ChangeLog:

	PR target/97865
	* configure: Regenerate.

libgomp/ChangeLog:

	PR target/97865
	* configure: Regenerate.

libhsail-rt/ChangeLog:

	PR target/97865
	* configure: Regenerate.

libobjc/ChangeLog:

	PR target/97865
	* configure: Regenerate.

libphobos/ChangeLog:

	PR target/97865
	* configure: Regenerate.

libquadmath/ChangeLog:

	PR target/97865
	* configure: Regenerate.

libssp/ChangeLog:

	PR target/97865
	* configure: Regenerate.

libstdc++-v3/ChangeLog:

	PR target/97865
	* configure: Regenerate.

libvtv/ChangeLog:

	PR target/97865
	* configure: Regenerate.

zlib/ChangeLog:

	PR target/97865
	* configure: Regenerate.