Support Linux on MIPS and ARMv5TE

[tatsuya6502]

Support the following 32-bit Linux platforms if possible:

• mipsel-unknown-linux-musl (for 麻烦编译一个mips版本的 messense/aliyundrive-webdav#22)
• armv5te-unknown-linux-musleabi (for 能不能帮忙编译个armv5版本 messense/aliyundrive-webdav#23)

Currently Moka will not compile by the following reasons:

• Moka uses std::atomic::AtomicU64 mainly as the expiration clock for entries, but AtomicU64 is not available on mipsel.
  • "PowerPC and MIPS platforms with 32-bit pointers do not have AtomicU64 or AtomicI64 types" (doc)
  • It seems AtomicU64 is not available on ARMv5TE too.
• quanta will not compile too because crossbeam-utils does not provide atomic::AtomicCell::fetch_add method on these platforms: (Does not build on MIPS and ARMv5 metrics-rs/quanta#54)

Probably we will replace all usages of AtomicU64 with RwLock<Option<std::time::Instant>> on these platforms?

[tatsuya6502]

Fixed this via #39.

[tatsuya6502]

@messense — Thank you for testing PR #39. I just published Moka v0.5.3 to crate.io.