libcxxabi: declare __gnu_unwind_frame in cxa_personality (#189787)

ARM EHABI builds of libcxxabi fail with clang-22+ because
cxa_personality.cpp calls __gnu_unwind_frame without a visible
declaration, triggering:

  error: use of undeclared identifier '__gnu_unwind_frame'

Add an extern "C" forward declaration before the EHABI unwind helper so
the source compiles correctly.

Signed-off-by: Khem Raj <khem.raj@oss.qualcomm.com>
NOKEYCHECK=True
GitOrigin-RevId: 8119f1854948b50358bbfaea08f207f51970f06c