testsuite: Require lto-plugin in gcc.dg/ipa/ipa-icf-38.c [PR85656]

testsuite: Require lto-plugin in gcc.dg/ipa/ipa-icf-38.c [PR85656]

gcc.dg/ipa/ipa-icf-38.c currently FAILs on Solaris (SPARC and x86, 32
and 64-bit):

FAIL: gcc.dg/ipa/ipa-icf-38.c scan-ltrans-tree-dump-not optimized "Function bar"

As it turns out, this only happens when the Solaris linker is used; with
GNU ld the test PASSes just fine.  In fact, that happens because gld
supports the lto-plugin while ld does not: in a Solaris build with gld,
the test FAILs the same way as with ld when -fno-use-linker-plugin is
passed, so this patch requires linker_plugin.

Tested on i386-pc-solaris2.11 (ld and gld) and x86_64-pc-linux-gnu.

2024-05-15  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc/testsuite:
	PR ipa/85656
	* gcc.dg/ipa/ipa-icf-38.c: Require linker_plugin.