www/qt6-webengine: Fix build on aarch64

../../../../../qtwebengine-everywhere-src-6.11.1/src/3rdparty/chromium/third_party/webrtc/rtc_base/cpu_info.cc:182:18:
error: use of undeclared identifier 'AT_HWCAP'

182 |     elf_aux_info(AT_HWCAP, &hwcap, sizeof(hwcap));
    |                  ^

../../../../../qtwebengine-everywhere-src-6.11.1/src/3rdparty/chromium/third_party/webrtc/rtc_base/cpu_info.cc:187:18:
error: use of undeclared identifier 'HWCAP_ASIMD'

187 |     if ((hwcap & HWCAP_ASIMD) != 0) {
    |                  ^

2 errors generated.

Reported by: pkg-fallout