-fsanitize=function: fix .subsections_via_symbols (#87527)

-fsanitize=function emits a signature and function hash before a
function. Similar to 7f6e2c97, these can be sheared off when
`.subsections_via_symbols` is used.

This change uses the same technique 7f6e2c97 introduced for prefixes:
emitting a symbol for the metadata, then marking the actual function
entry as an .alt_entry symbol.