[XRay] [compiler-rt] - Fix standalone and non-deterministic test issue

Summary:
The thread order test fails sometimes my machine independently of standalone
build.

From testing both standalone and in-tree build, I see I configured it wrong.

The other hypothesis for an issue is that cold starts can interfere with whether
record unwriting happens. Once this happens more than once, we can naively
FileCheck on the wrong test output, which compounds the issue.

While "rm blah.* || true" will print to stderr if the glob can't expand, this is
mostly harmless and makes sure earlier failing tests don't sabotage us.

Example failure:

---
header:
  version:         1
  type:            1
  constant-tsc:    true
  nonstop-tsc:     true
  cycle-frequency: 3800000000
records:
  - { type: 0, func-id: 1, function: 'f1()', cpu: 9, thread: 21377, kind: function-enter, tsc: 2413745203147228 }
  - { type: 0, func-id: 1, function: 'f1()', cpu: 9, thread: 21377, kind: function-exit, tsc: 2413745203304238 }
...

The CMAKE related change fixes the expectation that COMPILER_RT_STANDALONE_BUILD will be explicitly FALSE instead
of empty string when it is not "TRUE".

Reviewers: dberris

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32259

llvm-svn: 300822