lto: Fix option merging [PR106129]

lto: Fix option merging [PR106129]

The LTO merging of options from different input files was broken by:

commit 227a2ecf663d69972b851f51f1934d18927b62cd
Author: Martin Liska <mliska@suse.cz>
Date:   Fri Mar 12 11:53:47 2021 +0100

    lto-wrapper: Use vec<cl_decoded_option> data type.

Previously, find_and_merge_options would merge options it read into
those in *opts. After this commit, options in *opts on entry to
find_and_merge_options are ignored; the only merging that takes place
is between multiple sets of options in the same input file that are
read in the same call to this function (not sure how that case can
occur at all). The effects include, for example, that if some objects
are built with PIC enabled and others with it disabled, and the last
LTO object processed has PIC enabled, the choice of PIC for the last
object will result in the whole program being built as PIC, when the
merging logic is intended to ensure that a mixture of PIC and non-PIC
objects results in the whole program being built as non-PIC.

Fix this with an extra argument to find_and_merge_options to determine
whether merging should take place.  This shows up a second issue with
that commit (which I think wasn't actually intended to change code
semantics at all): once merging is enabled again, the check for
-Xassembler options became an infinite loop in the case where both
inputs had -Xassembler options, with the same first option, so fix
that loop to restore the previous semantics.

Note that I'm not sure how LTO option merging might be tested in the
testsuite (clearly there wasn't sufficient, if any, coverage to detect
these bugs).

Bootstrapped with no regressions for x86_64-pc-linux-gnu.

	PR lto/106129
	* lto-wrapper.cc (find_option): Add argument start.
	(merge_and_complain): Loop over existing_opt_index and
	existing_opt2_index for Xassembler check.  Update calls to
	find_option.
	(find_and_merge_options): Add argument first to determine whether
	to merge options with those passed in *opts.
	(run_gcc): Update calls to find_and_merge_options.

(cherry picked from commit 8a8ee37a3325f1009034245676ef4e482c0444a2)