testsuite: Fix gcc.dg/plugin/must-tail-call-2.c on SPARC [PR121159]

testsuite: Fix gcc.dg/plugin/must-tail-call-2.c on SPARC [PR121159]

The gcc.dg/plugin/must-tail-call-2.c test FAILs on 32 and 64-bit SPARC:

FAIL: gcc.dg/plugin/must-tail-call-2.c -fplugin=./must_tail_call_plugin.so (test for excess errors)

Excess errors:
gcc.dg/plugin/must-tail-call-2.c:58:3: error: cannot tail-call: target is not able to optimize the call into a sibling call

As discussed in the PR, this can be avoided by compiling with
-fdelayed-branch on SPARC.

Tested on sparc-sun-solaris2.11, sparc64-unknown-linux-gnu, and
x86_64-pc-linux-gnu.

2026-03-09  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc/testsuite:
	PR middle-end/121159
	* gcc.dg/plugin/must-tail-call-2.c (dg-additional-options): Add
	-fdelayed-branch on SPARC.