amend! [compiler-rt] Propagate sysroot from CMake to msan tests

[compiler-rt] Propagate sysroot from CMake to msan tests

Some msan tests requires rpc/xdr.h, which is no longer available
in some newer glibc. In build systems with hermetic sysroot set through
CMake, this issue can be solved by setting the sysroot to CMAKE_SYSROOT
for these tests. This patch implements it.

Author: zeroomega