LTO: Prune some warnings in the testsuite

Warnings like:

    warning: using serial compilation of 2 LTRANS jobs
    warning: visibility attribute not supported in this configuration; ignored\[^\n\]*" $text "" text

are pruned by lto.exp, but not for LTO testcases when run in other parts
of the testsuite. They will be, now.

gcc/testsuite/ChangeLog:

	PR testsuite/47334
	PR testsuite/103823
	* lib/prune.exp: Prune some warnings related to LTO and
	visibility.