[llvm-strings] Fix whitespaces to match strings output.

Summary:
The current implementation prepends a space on every line, making it difficult to compare against GNU strings.

The space appears to have come from handling --radix in rL292707. The space is for making sure there's a space between the radix and the value; however the space is still emitted even when there is no radix. This change fixes that so the space is only emitted when there is a radix.

Reviewers: jhenderson

Reviewed By: jhenderson

Subscribers: llvm-commits, compnerd

Differential Revision: https://reviews.llvm.org/D54238

llvm-svn: 346529