[ELF] Add --default-script/-dT

GNU ld added --default-script (alias: -dT) in 2007. The option specifies
a default script that is processed if --script/-T is not specified. -dT
can be used to override GNU ld's internal linker script, but only when
the application does not specify -T.
In addition, dynamorio's CMakeLists.txt may use -dT.

The implementation is simple and the feature can be useful to dabble
with different section layouts.

Pull Request: https://github.com/llvm/llvm-project/pull/89327