Fix _Unwind_Backtrace symbol version for ARM.

In real GNU libgcc, _Unwind_Backtrace is published with GCC_3.3 version
for all architectures but ARM. For ARM it's publishes with GCC_4.3.0
version.
This exception is not implement in your version of libggc, thus we
export _Unwind_Backtrace with bad version. To maintain backward
compatibility, publish _Unwind_Backtrace twice, once as compatible
symbol with GCC_3.3 version, and once as default symbol with
GCC_4.3.0 version.

While I'm in, fix typo in GCC_4.2.0 to GCC_4.3.0 inheritance declaration.

MFC after: 2 weeks