testsuite: Split spaceship_mixed_variants tests containing long long as input arg

The test spaceship_mixed_variants.c failed on ilp32 targets since the
targets will not have a DI mode spaceship optab, and thus the spaceship
optimization would not happen.
Separated out the tests that contain long long as input argument to the
spaceship optab to a different test file with appropriate target checks
done for the test.

2026-06-22  Avinash Jayakar  <avinashd@ibm.com>

gcc/testsuite/ChangeLog:
	* gcc.dg/spaceship_mixed_variants.c: Split long long arg tests
	in a different file.
	* gcc.dg/spaceship_mixed_variants_ll.c: New test.