Crash while building sysimage

Building an sysimage with Julia 1.4.2 on Ubuntu 20.04 fails with the following output:

```
julia> versioninfo()
Julia Version 1.4.2
Commit 44fa15b150* (2020-05-23 18:35 UTC)
Platform Info:
OS: Linux (x86_64-pc-linux-gnu)
CPU: Intel(R) Core(TM) i5-6300U CPU @ 2.40GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-8.0.1 (ORCJIT, skylake)
Environment:
JULIA_NUM_THREADS = 1

julia> using PackageCompiler

julia> PackageCompiler.create_sysimage([:ApproxFun,:Plots], precompile_execution_file="precompile_approxfun.jl", replace_default=true)
[ Info: PackageCompiler: creating system image object file, this might take a while...
Two passes with the same argument (-LowerPTLS) attempted to be registered!

signal (11): Segmentation fault
in expression starting at REPL[2]:1
_ZN4llvm14PassNameParser13passEnumerateEPKNS_8PassInfoE at /home/mocar/julia-1.4.2/bin/../lib/julia/libLLVM-8jl.so (unknown line)
Allocations: 11949240 (Pool: 11945748; Big: 3492); GC: 13

```

Reproduce the failure on Ubuntu 20.04 with Julia 1.4.2 using PackageCompiler.create_sysimage([:ApproxFun,:Plots], precompile_execution_file="precompile_approxfun.jl", replace_default=true). Start by examining the reported duplicate -LowerPTLS registration and LLVM stack trace. Done means the same sysimage command completes without the segmentation fault.

Written by the indexing model from the issue text.