Auto merge of #135764 - nikic:llvm-20-test-fixes, r=wesleywiser

Fix tests on LLVM 20

For sparcv8plus.rs, duplicate the test for LLVM 19 and LLVM 20. LLVM 20 resolves one of the FIXME in the test.

For x86_64-bigint-add.rs split the check lines for LLVM 19 and LLVM 20. The difference in codegen here is due to a difference in unroll factor, which I believe is not what the test is interested in.

Fixes rust-lang/rust#132957.
Fixes rust-lang/rust#133754.