Draft PGO+LTO+BOLT build + test CI

[Zentrik]

The build should be probably just need to build the default target instead of 7 different ones like in https://github.com/JuliaCI/julia-buildkite/pull/345/files, but I'll leave that for later (Zentrik/julia@88c90ca needs to be applied to pgo + lto+ bolt build).

[Zentrik]

First build is taking forever for some reason :(

[Zentrik]

Befuddled why llvm failed to build the second time with

-- Performing Test LLVM_LIBSTDCXX_MIN - Failed
CMake Error at cmake/modules/CheckCompilerVersion.cmake:88 (message):
  libstdc++ version must be at least 7.4.
Call Stack (most recent call first):
  cmake/config-ix.cmake:16 (include)
  CMakeLists.txt:981 (include)

[Zentrik]

Huh the hail Mary actually fixed it, weird. Failing to find bolt profiles now, probably not going to be able to look at this for a month if someone else wants to fix.

[Zentrik]

Befuddled why llvm failed to build the second time with

-- Performing Test LLVM_LIBSTDCXX_MIN - Failed
CMake Error at cmake/modules/CheckCompilerVersion.cmake:88 (message):
  libstdc++ version must be at least 7.4.
Call Stack (most recent call first):
  cmake/config-ix.cmake:16 (include)
  CMakeLists.txt:981 (include)

Looks like this was due to installing libstdc++ from CSL occuring whilst this check ran, so the compiler saw a partial libstdc++ and errored.
Manually worked around for now.

Not sure why a empty default.profraw (pgo profile) is being generated, will just manually remove for now.

[Zentrik]

Created a PR for the fix that bolt profiles of libjulia*.so were not being generated - JuliaLang/julia#59978.