Tier System: `mips(el)-linux`

[andrewrk]

Once all these items are complete, we can say that Zig has Tier 1 support for mips(el)-linux:

• Standard library support
• libc is available even when cross-compiling
• Stack traces work
• All behavior tests pass
• All standard library tests pass
• No disabled tests
• Enabled in CI (QEMU)

Open issues:

• Implement backtracing for MIPS #6690
• Re-enable FastISel for MIPS O32 with LLVM 20 #21215
• Remove "slow target" flag for mips(el)-linux targets in tests with LLVM 20 #21096
• behavior.vector.test.vector float operators causes LLVM assertion failure on mips*-linux-* #21051

[Ev1lT3rm1nal]

Task 4 is already fixed