[libc][NFC] Simplify assert message generation

Previously displaying a failed assert would involve a runtime integer to
string conversion. This patch changes that to be a compile time string
conversion.

This was inspired by a comment by JonChesterfield on https://reviews.llvm.org/D155899

Reviewed By: lntue, sivachandra, JonChesterfield

Differential Revision: https://reviews.llvm.org/D156168