[scudo][standalone] Correct cmake copy/paste error

Summary:
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

llvm-svn: 353528