Respect JULIA_CPU_TARGET when doing trimming (#57373)

We could make this have a nicer API but for now lets just do what we've
done for pkgimages/sysimages and use the envar

Author: gbaraldi

contrib/juliac.jl

@@ -6,6 +6,8 @@ module JuliaConfig
 end
 
 julia_cmd = `$(Base.julia_cmd()) --startup-file=no --history-file=no`
+cpu_target = get(ENV, "JULIA_CPU_TARGET", nothing)
+julia_cmd_target =  `$(Base.julia_cmd(;cpu_target)) --startup-file=no --history-file=no`
 output_type = nothing  # exe, sharedlib, sysimage
 outname = nothing
 file = nothing
@@ -102,7 +104,7 @@ end
 
 function compile_products()
     # Compile the Julia code
-    cmd = addenv(`$julia_cmd --project=$(Base.active_project()) --output-o $img_path --output-incremental=no --strip-ir --strip-metadata $julia_args $(joinpath(@__DIR__,"juliac-buildscript.jl")) $absfile $output_type $add_ccallables`, "OPENBLAS_NUM_THREADS" => 1, "JULIA_NUM_THREADS" => 1)
+    cmd = addenv(`$julia_cmd_target --project=$(Base.active_project()) --output-o $img_path --output-incremental=no --strip-ir --strip-metadata $julia_args $(joinpath(@__DIR__,"juliac-buildscript.jl")) $absfile $output_type $add_ccallables`, "OPENBLAS_NUM_THREADS" => 1, "JULIA_NUM_THREADS" => 1)
     verbose && println("Running: $cmd")
     if !success(pipeline(cmd; stdout, stderr))
         println(stderr, "\nFailed to compile $file")