www/firefox: unbreak with JIT (e.g. on powerpc*, sparc64)

In file included from js/src/builtin/RegExp.cpp:22:
In file included from js/src/jsobjinlines.h:33:
In file included from js/src/vm/TypeInference-inl.h:19:
In file included from js/src/jit/BaselineJIT.h:18:
In file included from js/src/jit/MacroAssembler.h:28:
js/src/jit/none/MacroAssembler-none.h:51:27: error: chosen constructor is

explicit in copy-initialization

static constexpr Register WasmIonExitTlsReg = { Registers::invalid_reg };

^                   ~~~~~~~~~~~~~~~~~~~~~~~~~~

js/src/jit/Registers.h:39:24: note: explicit constructor declared here

explicit constexpr Register(Encoding e)
                   ^

1 error generated.