Use x86_64-unknown-linux-gnu for host cargo build
* Now a host cargo build uses the same --target flag as a default
device build.
* When --device is given without --no-host or other device flag,
we can use one cargo build to get the required rustc flags for
both host and device builds.
* We still need merge_host_device because cargo build --test could
recompile a library and create duplicated calls to rustc.
* Fix a gpylint warning missing-function-docstring.
* This change also detected a bug in
external/rust/crates/structopt-derive/Android.bp
The cargo build --tests for device was optimized out
and the device test module was missing in Android.bp.
Bug: 169872957
Test: regenerate all .bp files in external/rust/crates
Change-Id: Ieabdc5a99534a60e4d459c9a966f7a867104c785
1 file changed