This fixes an internal error in GAS, triggered by the test case reported in PR 16765.

This fixes an internal error in GAS, triggered by the test case reported in PR 16765.
The problem was that gcc was generating assembler with missing unwind directives in it,
so that a gas_assert was being triggered.  The patch replaces the assert with an error
message.

	* config/tc-arm.c (create_unwind_entry): Report an error if an
	attempt to recreate an unwind directive is encountered.