I thought to report this upstream, but I cannot reproduce this with their build system.

To reproduce: in a WebRTC checkout:
gn gen out/linux-arm64-debug --args='target_cpu="arm64" is_debug=true'
ninja -C out/linux-arm64-debug

Although the build succeeds, towards the end it keeps repeating this worrisome line:
/usr/bin/aarch64-linux-gnu-ld: obj/third_party/protobuf/libprotobuf_lite.a(arena.o)(.debug_info+0x3f): R_AARCH64_ABS64 used with TLS symbol _ZN6google8protobuf5Arena13thread_cache_E

To me it seems like protobuf attempts to use some processor instruction which is not available on ARM64. I traced this to GOOGLE_PROTOBUF_NO_THREADLOCAL - if I force this macro to be defined unconditionally here then the error disappears:
https://chromium.googlesource.com/chromium/src/third_party/+/1be414c149bd29dc8e4be4b7db7efa459327fd68/protobuf/src/google/protobuf/stubs/platform_macros.h#117

I guess that some investigation has been done regarding a similar error https://github.com/google/protobuf/issues/347 but the root cause is probably not that the OS is IPHONE or ANDROID but the CPU architecture.

Reference: https://bugs.chromium.org/p/webrtc/issues/detail?id=7024