[Metadata] Add 'exclude' metadata to add the exclude flags on globals

This patchs adds a new metadata kind `exclude` which implies that the
global variable should be given the necessary flags during code
generation to not be included in the final executable. This is done
using the ``SHF_EXCLUDE`` flag on ELF for example. This should make it
easier to specify this flag on a variable without needing to explicitly
check the section name in the target backend.

Depends on D129053 D129052

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D129151