BLST compact unwind warning on mac

[mratsim]

libbacktrace gives the following warning when compiling any library (beacon_node, ncli_db, ...) on MacOS

ld: warning: could not create compact unwind for _sha256_block_data_order: does not use RBP or RSP based frame

This is non-blocking but will likely trigger questions from user.

The code that is pointed at is an assembly proc from BLST when libunwind tries to create a stacktrace for it.

[stefantalpalaru]

I don't think this has anything to do with libbacktrace or libunwind - both disabled on macOS, until this PR lands, BTW: #935

It's just the linker complaining about Assembly code in BLST that prevents it from generating smaller DWARF debugging info.

There is a flag to disable that compact format: -Wl,-no_compact_unwind but the Rust guys found it to be buggy on some old macOS version, and modified their Asm code instead to have the linker skip specific objects: rust-lang/rust#10013

The libffi folks went further and wrote their own compact unwinding sections in Asm: https://github.com/libffi/libffi/pull/440/files

All these Asm shenanigans are too complicated for us, so just use the compiler/linker flag instead.

[mratsim]

Fixed by #1789