Remove "slow target" flag for `mips(el)-linux` targets in tests

[alexrp]

Right now, we're flagging these targets as slow by default, causing them to not get run in tests unless -Dtest-slow-targets is passed. In practice, this means that most contributors will not run these tests, but CI will.

https://github.com/ziglang/zig/blob/11176d22f82861b4b6967b77f753414f214bc632/test/tests.zig#L313-L365

We do this because it takes 30-60 minutes to compile the tests for these targets due to a severe LLVM performance issue. The upstream issue is filed here: https://github.com/llvm/llvm-project/issues/104562

This issue is to track removing the "slow target" flag from these if/when the LLVM bug gets fixed.

[alexrp]

Bad news: https://github.com/ziglang/zig/pull/21125

Good news: https://github.com/llvm/llvm-project/pull/104723

[alexrp]

The LLVM 19 backport for this was rejected, so this will have to wait until LLVM 20.