rust cross compile to riscv64gc

[H-Y-B]

I want help, please.
(0)This is my rustc:
rustc 1.34.0 (91856ed52 2019-04-10)

(1) I have installed riscv64gc-unknown-none-elf.
`hyb:~/Rust/riscv64$ rustup show
Default host: x86_64-unknown-linux-gnu

installed targets for active toolchain

armv7-unknown-linux-gnueabihf
riscv64gc-unknown-none-elf
x86_64-unknown-linux-gnu

active toolchain

stable-x86_64-unknown-linux-gnu (default)
rustc 1.34.0 (91856ed52 2019-04-10)
`
(2) This is my ~/.cargo/config

[target.riscv64gc-unknown-none-elf] linker="riscv64-unknown-elf-gcc"
(3) Then, i run this command
cargo build --target=riscv64gc-unknown-none-elf

(4)But it shows this:
Compiling riscv64 v0.1.0 (/home/hyb/Rust/riscv64) error[E0463]: can't find crate for std| = note: theriscv64gc-unknown-none-elf` target may not be installed

error: aborting due to previous error

For more information about this error, try rustc --explain E0463.
error: Could not compile riscv64.

To learn more, run the command again with --verbose.
`
I want to know why? linker or std or rustc version?