handle unwind tables that are embedded within unwinding code [PR111731]

Original bug report: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111731

The unwinding mechanism registers both the code range and the unwind
table itself within a b-tree lookup structure. That data structure
assumes that is consists of non-overlappping intervals. This
becomes a problem if the unwinding table is embedded within the
code itself, as now the intervals do overlap.

To fix this problem we now keep the unwind tables in a separate
b-tree, which prevents the overlap.

libgcc/ChangeLog:
	PR libgcc/111731
	* unwind-dw2-fde.c: Split unwind ranges if they contain the
	unwind table.