Check for conflicting @ccallable name before JIT registration

This turns the existing segfault into an error:
```julia
julia> using Foo, Bar
ERROR: @ccallable: symbol 'foo' conflicts with existing symbol
Stacktrace:
  [1] _include_from_serialized(pkg::Base.PkgId, path::String, ocachepath::String, depmods::Vector{…}, ignore_native::Nothing; register::Bool)
    @ Base ./loading.jl:1241
  [2] _include_from_serialized (repeats 2 times)
    @ ./loading.jl:1206 [inlined]
  [3] _require_search_from_serialized(pkg::Base.PkgId, sourcepath::String, build_id::UInt128, stalecheck::Bool; reasons::Dict{…}, DEPOT_PATH::Vector{…})
    @ Base ./loading.jl:2018
  [4] _require(pkg::Base.PkgId, env::String)
    @ Base ./loading.jl:2477
  [5] __require_prelocked(uuidkey::Base.PkgId, env::String)
    @ Base ./loading.jl:2343
  [6] #invoke_in_world#2
    @ ./essentials.jl:1082 [inlined]
  [7] invoke_in_world
    @ ./essentials.jl:1079 [inlined]
  [8] _require_prelocked(uuidkey::Base.PkgId, env::String)
    @ Base ./loading.jl:2330
  [9] macro expansion
    @ ./loading.jl:2269 [inlined]
 [10] macro expansion
    @ ./lock.jl:287 [inlined]
 [11] __require(into::Module, mod::Symbol)
    @ Base ./loading.jl:2226
 [12] #invoke_in_world#2
    @ ./essentials.jl:1082 [inlined]
 [13] invoke_in_world
    @ ./essentials.jl:1079 [inlined]
 [14] require(into::Module, mod::Symbol)
    @ Base ./loading.jl:2219
Some type information was truncated. Use `show(err)` to see complete types.
```

Ideally the loading code would know to anticipate this and display a
better error to the user (w/o the useless stack-trace)

Resolves #54878, although we may wish to make this a warning or provide
an alternative `using` / `import` pathway that can allow `@ccallable`
registration to fail.

Author: topolarity

src/codegen.cpp

@@ -7619,12 +7619,15 @@ const char *jl_generate_ccallable(LLVMOrcThreadSafeModuleRef llvmmod, void *sysi
                 // restore a ccallable from the system image
                 void *addr;
                 int found = jl_dlsym(sysimg_handle, name, &addr, 0);
-                if (found)
-                    add_named_global(name, addr);
-                else {
-                    err = jl_get_exceptionf(jl_errorexception_type, "%s not found in sysimg", name);
+                if (!found) {
+                    err = jl_get_exceptionf(jl_errorexception_type, "@ccallable: symbol '%s' not found in sysimg", name);
+                    jl_throw(err);
+                }
+                if (jl_ExecutionEngine->getGlobalValueAddress(StringRef(name)) != 0) {
+                    err = jl_get_exceptionf(jl_errorexception_type, "@ccallable: symbol '%s' conflicts with existing symbol", name);
                     jl_throw(err);
                 }
+                add_named_global(name, addr);
             }
             else {
                 jl_method_instance_t *lam = jl_get_specialization1((jl_tupletype_t*)sigt, world, &min_valid, &max_valid, 0);