Compilers: Ignore -print-sysroot prefix when it is '/'

Since commit 8cc384f629 (Compilers: Add paths from -print-sysroot to
system prefix path, 2020-03-25, v3.18.0-rc1~337^2) we prepend the
compiler's sysroot to `CMAKE_SYSTEM_PREFIX_PATH`.  This does not
make sense when the prefix is just `/`, such as on Ubuntu 16.04's
system compiler.

Fixes: #21019
1 file changed