testsuite: Require lto-plugin support in gcc.dg/lto/modref-3 etc. [PR98237]

gcc.dg/lto/modref-3 etc. FAIL on Solaris with the native linker:

FAIL: gcc-dg-lto-modref-3-01.exe scan-wpa-ipa-dump modref "parm 1 flags: no_direct_clobber no_direct_escape"
FAIL: gcc-dg-lto-modref-4-01.exe scan-wpa-ipa-dump modref "parm 1 flags: no_direct_clobber no_direct_escape"
FAIL: gcc.dg/lto/modref-3 c_lto_modref-3_0.o-c_lto_modref-3_1.o execute -O2 -flto-partition=max -fdump-ipa-modref -fno-ipa-sra -fno-ipa-cp -flto
FAIL: gcc.dg/lto/modref-4 c_lto_modref-4_0.o-c_lto_modref-4_1.o execute -O2 -flto-partition=max -fdump-ipa-modref -fno-ipa-sra -flto

The issue is that the tests require the linker plugin, which isn't
available with Solaris ld.  Thus, it also FAILs when gcc is configured
with --disable-lto-plugin.

This patch thus declares the requirement.  As it turns out, there's an
undocumented dg-require-linker-plugin already, but I introduce and use
the corresponding effective-target keyword and document both.

Given that the effective-target form is more flexible, I'm tempted to
remove dg-require-* with an empty arg as already mentioned in
sourcebuild.texi.  That is not this patch, however.

Tested on i386-pc-solaris2.11 with ld and gld.

2024-02-14  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc/testsuite:
	PR ipa/98237
	* lib/target-supports.exp (is-effective-target): Handle
	linker_plugin.
	* gcc.dg/lto/modref-3_0.c: Require linker_plugin support.
	* gcc.dg/lto/modref-4_0.c: Likewise.

	gcc:
	* doc/sourcebuild.texi (Effective-Target Keywords, Other
	attribugs): Document linker_plugin.
	(Require Support): Document dg-require-linker-plugin.