grpcio: pre-generate bindings for common targets

[hunterlxt]

Building bindings at compile time introduces extra dependencies. And clang is hard to install and configure on some platform like centos 7. It will be very convenient that we can pre-generate bindings for common targets like x86_64-unknown-linux-gnu. For others, we can fallback to rust-bindgen at compile time.

[hunterlxt]

pre-bindings files could be different on different platforms and environments. Even in two computers in the same operating system environment, the order of the files is different. It may be difficult to use git diff to verify that the file has been modified. @BusyJay @nrc

[hunterlxt]

PLTA @BusyJay

[nrc]

@BusyJay do you have thoughts on #356 (comment) ?

[BusyJay]

there seems more risk here than there since the protobuf code is at least multi-platform.

That's why the bindings are named according to the target platform. It's intended to only maintain limited platform.

I agree we should remove the pre-generated bindings when rust-bindgen supports bundled clang. For now, pre-generating seems a more suitable solution.

[BusyJay]

Because clang is the default compiler on MacOS, so I drop the support. PTAL

[Connor1996]

LGTM