[clang] Allow -DDEFAULT_SYSROOT to be a relative path

In this case we interpret the path as relative the clang driver binary.

This allows SDKs to be built that include clang along with a custom
sysroot without requiring users to specify --sysroot to point to the
directory where they installed the SDK.

See https://github.com/WebAssembly/wasi-sdk/issues/58

Differential Revision: https://reviews.llvm.org/D76653