[PPC] Assertion failure in `disasm.cpp`

[vchuravy]

      From worker 7:	julia: /buildworker/worker/package_linuxppc64le/build/src/disasm.cpp:689: llvm::MCSymbol* {anonymous}::SymbolTable::lookupSymbol(uint64_t): Assertion `symb->isUndefined()' failed.
      From worker 7:	
      From worker 7:	signal (6): Aborted
      From worker 7:	in expression starting at /buildworker/worker/tester_linuxppc64le/build/share/julia/stdlib/v1.7/InteractiveUtils/test/runtests.jl:424
      From worker 7:	gsignal at /lib64/libc.so.6 (unknown line)
      From worker 7:	abort at /lib64/libc.so.6 (unknown line)
      From worker 7:	Allocations: 5060860003 (Pool: 5060132118; Big: 727885); GC: 1250
Worker 7 terminated.
InteractiveUtils                   (7) |         failed at 2021-02-18T03:02:04.770

as seen in https://build.julialang.org/#/builders/72/builds/843