restore non-freebsd-unix fix for profiling (#57249)

Restores #57035, undo #57089 for non-FreeBSD. While I suggested doing
this change for all platforms, I forgot that means non-FreeBSD platforms
become vulnerable again to the very deadlock problems that #57035 was
required to prevent. That fix seems to not be viable on FreeBSD due to
known libc implementation problems on that platform. However, upon
closer inspection of the questionable design implementation decisions
they seem to have made here, the platform is likely not currently
vulnerable to this libunwind bug in the first place:
https://github.com/lattera/freebsd/blob/master/libexec/rtld-elf/rtld_lock.c#L120

(cherry picked from commit 2f0a523)

src/signals-unix.c

@@ -310,14 +310,34 @@ int exc_reg_is_write_fault(uintptr_t esr) {
 #include <sys/eventfd.h>
 #include <link.h>
 
+#ifndef _OS_FREEBSD_
+typedef struct {
+    void (*f)(void*) JL_NOTSAFEPOINT;
+    void *ctx;
+} callback_t;
+static int with_dl_iterate_phdr_lock(struct dl_phdr_info *info, size_t size, void *data)
+{
+    jl_lock_profile();
+    callback_t *callback = (callback_t*)data;
+    callback->f(callback->ctx);
+    jl_unlock_profile();
+    return 1; // only call this once
+}
+#endif
+
 void jl_with_stackwalk_lock(void (*f)(void*), void *ctx)
 {
-    sigset_t sset, oset;
-    sigemptyset(&sset);
-    sigaddset(&sset, SIGUSR2);
-    pthread_sigmask(SIG_BLOCK, &sset, &oset);
+#ifndef _OS_FREEBSD_
+    callback_t callback = {f, ctx};
+    dl_iterate_phdr(with_dl_iterate_phdr_lock, &callback);
+#else
+    // FreeBSD makes the questionable decisions to use a terrible implementation of a spin
+    // lock and to block all signals while a lock is held. However, that also means it is
+    // not currently vulnerable to this libunwind bug that other platforms can encounter.
+    jl_lock_profile();
     f(ctx);
-    pthread_sigmask(SIG_SETMASK, &oset, NULL);
+    jl_unlock_profile();
+#endif
 }
 
 #if defined(_OS_LINUX_) && (defined(_CPU_X86_64_) || defined(_CPU_X86_))