[compiler-rt] [build_symbolizer] Introduce ZLIB_SRC to specify an on-disk location for (PR #70994)

[Fangrui Song via llvm-commits]

MaskRay wrote:

LGTM, but you may wait for @htyu who reported a similar problem yesterday and may verify whether this works for their system.

https://github.com/llvm/llvm-project/pull/70994