[scudo][standalone] Add string utility functions

Summary:
Add some string utility functions, notably to format strings, get
lengths, convert a string to a number. Those functions will be
used in reports and flags (coming up next). They were mostly borrowed
from sanitizer_common.

Make use of the string length function in a couple places in the
platform code that was checked in with inlined version of it.

Add some tests.

Reviewers: morehouse, eugenis, vitalybuka, hctim

Reviewed By: morehouse, vitalybuka

Subscribers: mgorny, delcypher, jdoerfert, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D59262

llvm-svn: 356457