[LNT] NFC. Improvements to cPerf
This patch contains few improvements to cPerf:

• Fixed the warning about llx printf format on LP64 systems
• Used environment variables in the standalone version for a debug purpose
• Added the comment with the command line to build the standalone version

OS Laboratory. Huawei Russian Research Institute. Saint-Petersburg

Reviewed By: thopre

Differential Revision: https://reviews.llvm.org/D117007