Bug#1016811: libwebkit2gtk-4.0-37: bullseye backport crashes a lot on arm64

[Loïc Yhuel]

The crash in operationArithNegateProfiledOptimize is due to a bug in
clang 10/11.
On AArch64, the frame pointer might be set incorrectly depending on
which registers are saved on the stack.
This is fixed in
https://github.com/llvm/llvm-project/commit/e9d9a61208, which applies
easily on clang 11.

WebKit uses __builtin_frame_address(1) in DECLARE_CALL_FRAME macro,
which due to the bug reads a saved floating point register, instead of
the frame pointer of the caller.