devel/llvm10: remove ELFv2 option to fix patch target

ELFv2 patches are now upstreamed and having them in ports actually breaks patch target:

<phase: patch >

> Patching for llvm10-10.0.0.r1

> Applying extra patch patches for llvm10-10.0.0.r1

> Applying extra patch patches for llvm10-10.0.0.r1

1 out of 1 hunks failed--saving rejects to tools/clang/lib/Basic/Targets/PPC.h.rej

> extra patch patch patch-tools_clang_lib_Basic_Targets_PPC.h failed to apply cleanly.

> Patch(es) patch-lib_Target_PowerPC_PPCTargetMachine.cpp applied cleanly.

Approved by: portmgr (fix build blanket)