Add SM4 cipher (GB/T 32907-2016) support from SM4 enabled mbed TLS library

[Low-power]

Hello.

There is a SM4 cipher support for mbed TLS in pull request Mbed-TLS/mbedtls#1165 some years. Unfortunately that pull request was still left unmerged and became outdated in current mbed TLS versions; however I still managed to merge that pull request into the long term support branch 2.7, and with this minor modification to shadowsocks-libev, the SM4 ciphers appears fully functional in shadowsocks-libev.
The implemented ciphers in shadowsocks-libev are sm4-128-cbc, sm4-128-ctr and sm4-128-gcm.

Until Mbed-TLS/mbedtls#1165 can be merged, mbed TLS woudn't offically supporting SM4 cipher, however I opened this pull request for discuss whether to check a downstream version of mbed TLS of this SM4 support, in shadowsocks-libev.

My mbed TLS branch with SM4 support merged is available at https://git.nsscn.top/Low-power/mbedtls/tree/mbedtls-2.7-sm4

[madeye]

For any cipher change, please go to https://github.com/shadowsocks/shadowsocks-org/issues

[ttimasdf]

Using SM4 for anti-censoring...This PR could definitely piss off those officers of OSCCA 🤣

[ghost]

anti-censoring

Maybe anti-(NSA's)censoring🤣

[Low-power]

Since the old repository I hosted my Mbed TLS fork has long gone, I republished the repository at https://github.com/Low-power/mbedtls/tree/mbedtls-2.7-sm4 for anyone interested in that.

[madeye]

Thanks for the SM4 (GB/T 32907-2016) support. This has drifted and now conflicts with master: it still patches the removed m4/ autotools files (the build is CMake-only now). Could you rebase onto current master and drop the m4/ changes? Keeping open as an enhancement.

[Low-power]

I didn't find the mbed TLS cipher availability checks in current CMake scripts, as previously implemented in m4/mbedtls.m4. I added only the SM4 cipher availability checking there, so I think this file can simply be removed without changing of CMake scripts.