[ELF] --sysroot: change sysrooted script to not fall back for an absolute path

Modify the D13209 logic: for a script inside the sysroot, if an absolute path
does not exist, report an error instead of falling back to the path without the
sysroot prefix.

This matches GNU ld, which makes sense to me: we don't want to find an arbitrary
file in the host.

Reviewed By: ikudrin

Differential Revision: https://reviews.llvm.org/D104894