[scudo][standalone] Correct cmake copy/paste error
I mistakenly used a SCUDO variable instead of SCUDO_STANDALONE one. The net
result was that there were more architecture supported than I intended.
Correct that, I'll add more architectures once the bots are enabled and
green for x86.
Reviewers: eugenis, vitalybuka
Reviewed By: vitalybuka
Subscribers: mgorny, delcypher, #sanitizers, llvm-commits
Tags: #llvm, #sanitizers
Differential Revision: https://reviews.llvm.org/D57897