www/firefox-esr: unbreak on powerpc64 after r472833

In file included from objdir/js/src/Unified_cpp_js_src41.cpp:2:
js/src/wasm/WasmSignalHandlers.cpp: In function 'uint8_t** ContextToPC(ucontext_t*)':
js/src/wasm/WasmSignalHandlers.cpp:439:19: error: 'R32_sig' was not declared in this scope
#define PC_sig(p) R32_sig(p)

^~~~~~~

js/src/wasm/WasmSignalHandlers.cpp:450:39: note: in expansion of macro 'PC_sig'

return reinterpret_cast<uint8_t**>(&PC_sig(context));
                                    ^~~~~~

js/src/wasm/WasmSignalHandlers.cpp:439:19: note: suggested alternative: 'R12_sig'
#define PC_sig(p) R32_sig(p)

^~~~~~~

js/src/wasm/WasmSignalHandlers.cpp:450:39: note: in expansion of macro 'PC_sig'

return reinterpret_cast<uint8_t**>(&PC_sig(context));
                                    ^~~~~~

Reported by: pkg-fallout