[LoongArch] Support -march=native and -mtune=

As described in [1][2], `-mtune=` is used to select the type of target
microarchitecture, defaults to the value of `-march`. The set of
possible values should be a superset of `-march` values. Currently
possible values of `-march=` and `-mtune=` are `native`, `loongarch64`
and `la464`.

D136146 has supported `-march={loongarch64,la464}` and this patch adds
support for `-march=native` and `-mtune=`.

A new ProcessorModel called `loongarch64` is defined in LoongArch.td
to support `-mtune=loongarch64`.

`llvm::sys::getHostCPUName()` returns `generic` on unknown or future
LoongArch CPUs, e.g. the not yet added `la664`, leading to
`llvm::LoongArch::isValidArchName()` failing to parse the arch name.
In this case, use `loongarch64` as the default arch name for 64-bit
CPUs.

And these two preprocessor macros are defined:
- __loongarch_arch
- __loongarch_tune

[1]: https://github.com/loongson/LoongArch-Documentation/blob/2023.04.20/docs/LoongArch-toolchain-conventions-EN.adoc
[2]: https://github.com/loongson/la-softdev-convention/blob/v0.1/la-softdev-convention.adoc

Differential Revision: https://reviews.llvm.org/D155824

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