Fixed: error: unnecessary trailing semicolon

[n-salim]

The latest bootstrap cargo (2019-08-13) caught a hitherto unnoticed issue:

error: unnecessary trailing semicolon
--> src/libstd/sys/vxworks/process/process_common.rs:158:26
|
158 | unimplemented!();;
| ^ help: remove this semicolon
|
= note: -D redundant-semicolon implied by -D warnings

error: unreachable statement
--> src/libstd/sys/vxworks/process/process_common.rs:158:26
|
158 | unimplemented!();;
| ^
|
= note: -D unreachable-code implied by -D warnings

error: aborting due to 2 previous errors

error: Could not compile std.

[BaoshanPang]

@n-salim

I created the PR for upstream: rust-lang#63704