Add cpu value loong64 for LoongArch64

[znley]

We intend to use loong64 as cpu constraint for LoongArch64.

Signed-off-by: znley [email protected]

[znley]

@xen0n

[znley]

@aiuto

[aiuto]

This is premature. From my quick research, it is not yet clear what the name will be in other projects, such as llvm. Most uses seem to be leaning towards 'loongarch64', as in https://wiki.debian.org/Ports/loongarch64 But that points to almost everything being still in the proposed state.

Let's do this again after a single name is adopted by the rest of the community, then we'll come back and add a matching one for Bazel.

[zhaixiaojuan]

@aiuto Hi, aiuto. Can this PR be reopened and considered for merging? At present, llvm, go, gcc, kernel and other communities have merged the code of the LoongArch architecture. Here we use the name loong64, mainly to maintain the consistency of other Google projects in naming the LoongArch architecture (such as go, v8, etc. all use loong64), and the current debian (https://wiki.debian.org/ports/loong64) community The name adopted is also loong64.