[ELF] Fix unnecessary inclusion of unreferenced provide symbols

Previously, linker was unnecessarily including a PROVIDE symbol which
was referenced by another unused PROVIDE symbol. For example, if a
linker script contained the below code and 'not_used_sym' provide symbol
is not included, then linker was still unnecessarily including 'foo' PROVIDE
symbol because it was referenced by 'not_used_sym'. This commit fixes
this behavior.

PROVIDE(not_used_sym = foo)
PROVIDE(foo = 0x1000)

This commit fixes this behavior by using dfs-like algorithm to find
all the symbols referenced in provide expressions of included provide
symbols.

This commit also fixes the issue of unused section not being garbage-collected
if a symbol of the section is referenced by an unused PROVIDE symbol.

Closes #74771
Closes #84730

Co-authored-by: Fangrui Song <i@maskray.me>