allow the Compiler.jl stdlib to be installed on older version of Julia

Since #56409, Compiler.jl as a standard library has
become available. However, for Julia versions prior to this change, even
though the stdlib can be installed via Pkg.jl, the precompilation fails
due to code compatibility issues. Consequently, when an external package
that uses the Compiler stdlib adds Compiler.jl to its Project.toml, the
package would stop working on older Julia versions.

To address this, this commit adopts the same approach as JET.jl.
Specifically, on older Julia versions, a dummy `Compiler` module is
defined, allowing dependent packages to switch between using the
Compiler.jl stdlib or the previous `Core.Compiler`. While this is a
somewhat hacky solution, it should resolve the issue for now.

Compiler/src/Compiler.jl

@@ -1,15 +1,26 @@
 # This file is a part of Julia. License is MIT: https://julialang.org/license
 
+if !isdefined(Base, :__Compiler_as_stdlib_available__)
+
+@eval module Compiler
+    function __init__()
+        println("""
+        The `Compiler` standard library is not available for this version of Julia.
+        Use Julia version `v"1.12.0-DEV.1581"` or later.
+        """)
+    end
+end
+
 # When generating an incremental precompile file, we first check whether we
 # already have a copy of this *exact* code in the system image. If so, we
 # simply generates a pkgimage that has the dependency edges we recorded in
 # the system image and simply returns that copy of the compiler. If not,
 # we proceed to load/precompile this as an ordinary package.
-if isdefined(Base, :generating_output) && Base.generating_output(true) &&
+elseif (isdefined(Base, :generating_output) && Base.generating_output(true) &&
         Base.samefile(joinpath(Sys.BINDIR, Base.DATAROOTDIR, Base._compiler_require_dependencies[1][2]), @eval @__FILE__) &&
         !Base.any_includes_stale(
             map(Base.compiler_chi, Base._compiler_require_dependencies),
-            "sysimg", nothing)
+            "sysimg", nothing))
 
     Base.prepare_compiler_stub_image!()
     append!(Base._require_dependencies, map(Base.expand_compiler_path, Base._compiler_require_dependencies))

base/Base_compiler.jl

@@ -288,6 +288,14 @@ process_sysimg_args!()
 
 function isready end
 
+"""
+    Base.__Compiler_as_stdlib_available__
+
+This is a token for switching the definition of `Compiler` to make it installable even on
+Julia versions where Compiler.jl is not available as a standard library.
+"""
+const __Compiler_as_stdlib_available__ = true
+
 include(strcat(BUILDROOT, "../usr/share/julia/Compiler/src/Compiler.jl"))
 
 const _return_type = Compiler.return_type