[gdb/testsuite] Require DW_LNE_end_sequence

The dwarf standard requires that every line number program sequence ends
with a DW_LNE_end_sequence instruction.

Enforce this in the dwarf assembler for the last sequence in a line number
program (we have no means to enforce this for earlier sequences), and fix a
few test-case that don't have it.

Tested on aarch64-linux.

PR testsuite/31618
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31618