Tier System: `loongarch64-linux`

[alexrp]

• 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)
• CI-produced binaries on download page

Open issues:

• zig libc crashes with out of memory error on loongarch64 #21468
• Fix loongarch64 ABI support for C procedure call #21671
• Remove the LoongArch f16 lowering workaround with LLVM 20 #22003

Open PRs:

• runtime page size detection + rework GeneralPurposeAllocator to reduce active mapping count + Allocator VTable API update #20511

[alexrp]

Changing the milestone because of: #23495 (comment)