Prefix execroot-relative file arguments with pwd

[martin4861]

Prefix execroot-relative bare positional file args with pwd in cargo build scripts. This is already done for -L, -isystem, --sysroot, etc but not for bare positional arguments like bazel-out/bin/compiler-rt/libclang_rt.builtins.static.a.

This problem arised when compiling aws-lc-sys with hermetic-llvm, see hermeticbuild/hermetic-llvm#405

A minimal working example to reproduce is created in https://github.com/martin4861/mwe-bootstrapped-rust-clang_rt.builtins.static:

clang: error: no such file or directory:
'bazel-out/darwin_arm64-fastbuild-ST-bdec89fd5d65/bin/external/llvm++llvm_source+compiler-rt/clang_rt.builtins.static_/libclang_rt.builtins.static.a'