make stacktrace lookup and unwinding more threadsafe

[vtjnash]

in particular, this is broken on Windows due to the jl_in_stackwalk global variable

[vchuravy]

I observed this failure on CI.

Assertion failed: !jl_in_stackwalk, file /cygdrive/d/buildbot/worker/package_win32/build/src/stackwalk.c, line 77
signal (22): SIGABRT
in expression starting at D:\buildbot\worker-tabularasa\tester_win32\build\share\julia\test\threads_exec.jl:538
Allocations: 8837471 (Pool: 8837213; Big: 258); GC: 63
threads: Error During Test at D:\buildbot\worker-tabularasa\tester_win32\build\share\julia\test\testdefs.jl:22
  Got exception outside of a @test
  LoadError: failed process: Process(setenv(`'D:\buildbot\worker-tabularasa\tester_win32\build\bin\julia.exe' -Cnative '-JD:\buildbot\worker-tabularasa\tester_win32\build\lib\julia\sys.dll' -g1 --depwarn=error --startup-file=no threads_exec.jl`,["JULIA_NUM_THREADS=4"]), ProcessExited(3)) [3]
  
  Stacktrace:
   [1] pipeline_error at .\process.jl:525 [inlined]
   [2] run(::Base.CmdRedirect; wait::Bool) at .\process.jl:440
   [3] run(::Base.CmdRedirect) at .\process.jl:438
   [4] top-level scope at D:\buildbot\worker-tabularasa\tester_win32\build\share\julia\test\threads.jl:5
   [5] include(::Function, ::Module, ::String) at .\Base.jl:382
   [6] include at .\Base.jl:370 [inlined]
   [7] include at D:\buildbot\worker-tabularasa\tester_win32\build\share\julia\test\testdefs.jl:13 [inlined]
   [8] macro expansion at D:\buildbot\worker-tabularasa\tester_win32\build\share\julia\test\testdefs.jl:25 [inlined]
   [9] macro expansion at D:\buildbot\worker\package_win32\build\usr\share\julia\stdlib\v1.5\Test\src\Test.jl:1114 [inlined]
   [10] macro expansion at D:\buildbot\worker-tabularasa\tester_win32\build\share\julia\test\testdefs.jl:24 [inlined]
   [11] macro expansion at .\timing.jl:310 [inlined]
   [12] top-level scope at D:\buildbot\worker-tabularasa\tester_win32\build\share\julia\test\testdefs.jl:22
   [13] eval at .\boot.jl:331 [inlined]
   [14] runtests(::String, ::String, ::Bool; seed::UInt128) at D:\buildbot\worker-tabularasa\tester_win32\build\share\julia\test\testdefs.jl:28
   [15] (::var"#30#41"{String})() at D:\buildbot\worker-tabularasa\tester_win32\build\share\julia\test\runtests.jl:261
   [16] top-level scope at none:1
   [17] eval at .\boot.jl:331 [inlined]
   [18] eval(::Expr) at .\client.jl:452
   [19] (::var"#23#32")() at D:\buildbot\worker-tabularasa\tester_win32\build\share\julia\test\runtests.jl:261
   [20] cd(::var"#23#32", ::String) at .\file.jl:93
   [21] top-level scope at D:\buildbot\worker-tabularasa\tester_win32\build\share\julia\test\runtests.jl:75
   [22] include(::Function, ::Module, ::String) at .\Base.jl:382
   [23] include(::Module, ::String) at .\Base.jl:370
   [24] exec_options(::Base.JLOptions) at .\client.jl:289
   [25] _start() at .\client.jl:491
  in expression starting at D:\buildbot\worker-tabularasa\tester_win32\build\share\julia\test\threads.jl:3