[sanitizer] Implement a __asan_default_options() equivalent for Scudo

Summary:
Currently, the Scudo Hardened Allocator only gets its flags via the SCUDO_OPTIONS environment variable.
With this patch, we offer the opportunity for programs to define their own options via __scudo_default_options() which behaves like __asan_default_options() (weak symbol).
A relevant test has been added as well, and the documentation updated accordingly.
I also used this patch as an opportunity to rename a few variables to comply with the LLVM naming scheme, and replaced a use of Report with dieWithMessage for consistency (and to avoid a callback).

Reviewers: llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D23018

llvm-svn: 277536