Set -fno-gnu-unique for building LLVM

[vchuravy]

On my system that uses a Radeon GPU the codesnippet below (from #37200)

using GLFW, ModernGL
window = GLFW.Window(resolution = (10, 10), major = 3, minor = 3, visible = false, focus = false)

crashes with

Thread 15 "julia:sh0" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x7fff827fc640 (LWP 2166709)]
0x00007fff89ea919d in llvm::TargetPassConfig::addPass(llvm::Pass*, bool, bool)
    () from /usr/lib/libLLVM-11.so
(gdb) bt
#0  0x00007fff89ea919d in llvm::TargetPassConfig::addPass(llvm::Pass*, bool, bool) () from /usr/lib/libLLVM-11.so
#1  0x00007fff89eabfc8 in llvm::TargetPassConfig::addRegAssignmentOptimized() () from /usr/lib/libLLVM-11.so
#2  0x00007fff89eac38e in llvm::TargetPassConfig::addOptimizedRegAlloc() () from /usr/lib/libLLVM-11.so
#3  0x00007fff89eac986 in llvm::TargetPassConfig::addMachinePasses() () from /usr/lib/libLLVM-11.so
#4  0x00007fff89bfb5d2 in ?? () from /usr/lib/libLLVM-11.so
#5  0x00007fff89bff8a1 in llvm::LLVMTargetMachine::addPassesToEmitFile(llvm::legacy::PassManagerBase&, llvm::raw_pwrite_stream&, llvm::raw_pwrite_stream*, llvm::CodeGenFileType, bool, llvm::MachineModuleInfoWrapperPass*) ()
   from /usr/lib/libLLVM-11.so
#6  0x00007fff8f1c6f66 in ?? () from /usr/lib/dri/radeonsi_dri.so
#7  0x00007fff8f107475 in ?? () from /usr/lib/dri/radeonsi_dri.so
#8  0x00007fff8f14f22b in ?? () from /usr/lib/dri/radeonsi_dri.so
#9  0x00007fff8ed8dd08 in ?? () from /usr/lib/dri/radeonsi_dri.so
#10 0x00007fff8ed8c418 in ?? () from /usr/lib/dri/radeonsi_dri.so
#11 0x00007ffff7f6c3e9 in start_thread () from /usr/lib/libpthread.so.0
#12 0x00007ffff7e9a293 in clone () from /usr/lib/libc.so.6
(gdb) 

The last time I dug into this it tourned out to be a problem with how gcc handles static variables
in particular it annotating them with GNU_UNIQUE and glibc promptly ignoring symbol versions of
symbols marked such.

Upstream bug report: https://bugs.llvm.org/show_bug.cgi?id=48221
I never got a reply on libc-help when I asked if that behaviour is intended https://sourceware.org/pipermail/libc-help/2020-November/005559.html

On the LLVM discord I got the recommendation to build LLVM with -fno-gnu-unique

cc: @wsmoses in case this impacts LLVM plugins

If this looks okay, I will fix this in Yggdrasil as well.

[Wikunia]

Just in time for me ☺️ thanks a lot @vchuravy
This solved my AMD problem with Makie!

[vchuravy]

@vtjnash any concerns? Otherwise I would like to merge this.

[vchuravy]

@vtjnash you asked me whether or not Clang tests are passing with this flag. I build LLVM ToT with -fno-gnu-unique and all the Clang tests are passing.

[vchuravy]

I am going to merge this, since it fixes a long-standing usability issue on Linux, but we should keep an eye out for issues that might happen with Clang.jl/Cxx.jl