Adds a convenient way to enable PGO+LTO on Julia and LLVM together:

1. `cd contrib/pgo-lto`
2. `make -j$(nproc) stage1`
3. `make clean-profiles`
4. `./stage1.build/julia -O3 -e 'using Pkg; Pkg.add("LoopVectorization"); Pkg.test("LoopVectorization")'`
5. `make -j$(nproc) stage2`

This results quite often in spectacular speedups for time to first X as
it reduces the time spent in LLVM optimization passes by 25 or even 30%.

Example 1:

```julia
using LoopVectorization
function f!(a, b)
    @turbo for i in eachindex(a)
        a[i] *= b[i]
    end
    return a
end
f!(rand(1), rand(1))
```

```console
$ time ./julia -O3 lv.jl
```

Without PGO+LTO: 14.801s
With PGO+LTO: 11.978s (-19%)

Example 2:

```console
$ time ./julia -e 'using Pkg; Pkg.test("Unitful");'
```

Without PGO+LTO: 1m47.688s
With PGO+LTO: 1m35.704s (-11%)

Example 3 (taken from issue JuliaLang#45395, which is almost only LLVM):

```console
$ JULIA_LLVM_ARGS=-time-passes ./julia script-45395.jl
```

Without PGO+LTO:

```
===-------------------------------------------------------------------------===
                      ... Pass execution timing report ...
===-------------------------------------------------------------------------===
  Total Execution Time: 101.0130 seconds (98.6253 wall clock)

   ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
  53.6961 ( 54.7%)   0.1050 (  3.8%)  53.8012 ( 53.3%)  53.8045 ( 54.6%)  Unroll loops
  25.5423 ( 26.0%)   0.0072 (  0.3%)  25.5495 ( 25.3%)  25.5444 ( 25.9%)  Global Value Numbering
   7.1995 (  7.3%)   0.0526 (  1.9%)   7.2521 (  7.2%)   7.2517 (  7.4%)  Induction Variable Simplification
   5.0541 (  5.1%)   0.0098 (  0.3%)   5.0639 (  5.0%)   5.0561 (  5.1%)
   Combine redundant instructions JuliaLang#2
```

Wit PGO+LTO:

```
===-------------------------------------------------------------------------===
                      ... Pass execution timing report ...
===-------------------------------------------------------------------------===
  Total Execution Time: 72.6507 seconds (70.1337 wall clock)

   ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
  36.0894 ( 51.7%)   0.0825 (  2.9%)  36.1719 ( 49.8%)  36.1738 ( 51.6%)  Unroll loops
  16.5713 ( 23.7%)   0.0129 (  0.5%)  16.5843 ( 22.8%)  16.5794 ( 23.6%)  Global Value Numbering
   5.9047 (  8.5%)   0.0395 (  1.4%)   5.9442 (  8.2%)   5.9438 (  8.5%)  Induction Variable Simplification
   4.7566 (  6.8%)   0.0078 (  0.3%)   4.7645 (  6.6%)   4.7575 (  6.8%)  Combine redundant instructions JuliaLang#2
```

Or -28% time spent in LLVM.

---

Finally there's a significant reduction in binary sizes. For libLLVM.so:

```
79M	usr/lib/libLLVM-13jl.so (before)
67M	usr/lib/libLLVM-13jl.so (after)
```

And it can be reduced by another 2MB with `--icf=safe` when using LLD as
a linker anways.

Turn into makefile

Newline

Use two out of source builds

Ignore profiles + build dirs

Add --icf=safe

stage0 setup prebuilt clang with [cd]tors->init/fini patch