[compiler-rt][test] Ensure CMAKE_SYSROOT is added as a test cflag if provided

When running tests like SanitizerCommon-asan-x86_64-Linux :: Linux/crypt_r.cpp,
it may attempt to use the host header crypt.h rather than a sysroot header.
This is significant in the event where struct crypt_data defined on host is
different from the sysroot used to make the sanitizer runtime libraries. This
can result in logical differences between the expected size/layout of struct
crypt_data known by sanitizers and the strict crypt_data provided by the host crypt.h.

Since tests should still use the CMAKE_SYSROOT, this ensures that CMAKE_SYSROOT
is propagated to compiler-rt tests.

Differential Revision: https://reviews.llvm.org/D109796