Disallow --incremental with -pie and force -no-pie for incremental tests.

Disallow --incremental with -pie and force -no-pie for incremental tests.

This is a partial fix for the gold testsuite failures documented in
PR 21090. The use of -fpie triggers some mov-to-lea optimizations that
are not compatible with incremental linking, so those optimizations need
to be disabled. We also diagnose the attempt to use -pie with incremental
linking, and force -no-pie for the incremental tests in case the build has
been configured to have GCC pass -pie all the time.

We still have a problem where compiling with -fpie results in some GOT
entries even when linking with -no-pie. This combination still causes test
failures because we are not updating the GOT entries in an incremental update
link.

gold/
	PR gold/21090
	* incremental.cc (Sized_relobj_incr::do_relocate): Fix comment.
	* options.cc (General_options::finalize): Disallow -pie with
	incremental linking.
	* x86_64.cc (Target_x86_64::Scan::local): Don't do mov-to-lea
	or callq-to-direct optimizations for incremental links.
	(Target_x86_64::Scan::global): Likewise.
	(Target_x86_64::Relocate::relocate): Likewise.
	* testsuite/Makefile.am (incremental_test): Force -no-pie.
	(incremental_test_2): Likewise.
	(incremental_test_3): Likewise.
	(incremental_test_4): Likewise.
	(incremental_test_5): Likewise.
	(incremental_test_6): Likewise.
	(incremental_copy_test): Likewise.
	(incremental_common_test_1): Likewise.
	(incremental_comdat_test_1):  Likewise.
	* testsuite/Makefile.in: Regenerate.