Update init.c for error on --handle-signals=no with multiple threads

[arnavk23]

This patch adds a check during startup to disallow using --handle-signals=no when multiple threads are configured via JULIA_NUM_THREADS > 1. This avoids segmentation faults due to the GC failing to safepoint properly without signal handling, as detailed in #50278.

[vchuravy]

Thanks! No need to open a new PR (x-ref #58463), you could have pushed a new version of your changes.

[arnavk23]

Yeah but I actually made a mistake while pushing and instead pushed a new pull request. I have closed the previous one.

[giordano]

analyzegc is unhappy:

/cache/build/builder-amdci4-7/julialang/julia-master/src/init.c:560:39: error: Implicit Atomic seq_cst synchronization [concurrency-implicit-atomics,-warnings-as-errors]
  560 |     if (!jl_options.handle_signals && jl_n_threads > 1) {
      |                                       ^
make: *** [Makefile:571: clang-tidy-init] Error 1

[arnavk23]

So what should I do

[gbaraldi]

look for how it's used elsewhere, you need to use a jl_atomic_load_relaxed there

[arnavk23]

@gbaraldi Corrected the file..