Don't pass --target in cargo.sh

[bjorn3]

This was a workaround for compiling proc macros resulting in an abi
incompatibility. By passing --target proc macros will be built by the
llvm backend. This is no longer necessary as the abi incompatibility has
since been fixed.

[bjorn3]

On further thought this will regress error messages when proc macros panic as proc macros will now be built with -Cpanic=abort. See rust-lang/rustc_codegen_cranelift#1174. @antoyo I will let it up to you if you want to merge this before implementing unwinding or if you want to wait on this.

[antoyo]

I'm ok with merging this without unwinding support.