paths passed to build script by runner that are relative to the `execroot` cause build script to fail

I have a custom toolchain that builds it's own `libc++` and sets a dependency from the toolchain to said `libc++`, and sets `execroot`-relative `-isystem` flags. When these are passed to a build script, it fails as the build script is not run from inside the execroot.

It appears that some other flags with this problem like `--sysroot` have special-cased handling, but I'm not sure that manually keeping track of compiler flags that may contain `execroot`-relative paths and replacing them with `{pwd}` for the build script runner is a long-term solution here, but I don't have other ideas.