[XRay][compiler-rt] Remove dependency on <system_error>

Summary:
Depending on C++11 <system_error> introduces a link-time requirement to
C++11 symbols. Removing it allows us to depend on header-only C++11 and
up libraries.

Partially fixes http://llvm.org/PR32274 -- we know there's more invasive work
to be done, but we're doing it incrementally.

Reviewers: dblaikie, kpw, pelikan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31233

llvm-svn: 298480