Fix build_sysimg on windows

[musm]

Note rm(....) will not work it errors with

ERROR: unlink: operation not permitted (EPERM)
 in uv_error at .\libuv.jl:68 [inlined]
 in unlink(::String) at .\file.jl:382
 in #rm#7(::Bool, ::Bool, ::Function, ::String) at .\file.jl:107
 in link_sysimg(::String, ::String, ::Bool) at C:\Julia\julia-0.5-latest\share\julia\build_sysimg.jl:168
 in (::##2#3{Bool,String})() at C:\Julia\julia-0.5-latest\share\julia\build_sysimg.jl:90
 in cd(::##2#3{Bool,String}, ::String) at .\file.jl:48
 in #build_sysimg#1(::Bool, ::Bool, ::Function, ::Void, ::String, ::Void) at C:\Julia\julia-0.5-latest\share\julia\build_sysimg.jl:48
 in (::#kw##build_sysimg)(::Array{Any,1}, ::#build_sysimg, ::Void, ::String, ::Void) at .\<missing>:0 (repeats 2 times)

Can't delete old dll so here we simply rename and leave as is.

[tkelman]

this should check if the source exists first

(also spaces after commas and semicolons are usually preferred for readability)

[musm]

thanks fixed

[musm]

This can probably be backported too

[ararslan]

This can probably be backported too

Just as a heads up, typically the release overlords (e.g. @tkelman) decide which PRs are candidates for backporting to previous releases, and which releases they'll be backported to.

[tkelman]

I don't mind people making suggestions though. Won't always agree, but if you've tested this in several use cases, then makes sense to me.

[tkelman]

I'm a little surprised this doesn't error, even after the file has been moved it's still loaded by the running executable

[musm]

Maybe. I have seen many times in windows that a stubborn file that refuses to be deleted just needs a rename, before you can delete it, strange but it work.

[musm]

Upon further testing using rm or del won't work. Simply renaming is the best/simplest option. Should be g2g now. Checked on cmd and powershell.

[musm]

closes #18377