[scudo][standalone] Do not error out on spurious C(XX) flags

Summary:
The standalone Scudo version is being built with `-Werror` which can be
tripped by extraneous command line arguments. We have little control over
those as they can be passed down to us by `CMAKE_C(XX)_FLAGS`, the reported
scenario involving `-stdlib=libc++` (see https://reviews.llvm.org/D57412#1384504).

To work around this, disable `-Wunused-command-line-argument`.

Reviewers: eugenis, vitalybuka, Eugene.Zelenko

Reviewed By: eugenis

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

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D57757

llvm-svn: 353418