gdb: Support DW_LLE_start_end

Without that it is impossible to debug on riscv64.

gdb/
	PR symtab/27999
	* dwarf2/loc.c (decode_debug_loclists_addresses): Support
	DW_LLE_start_end.

gdb/testsuite/
	PR symtab/27999
	* lib/dwarf.exp (start_end): New proc inside loclists.
	* gdb.dwarf2/loclists-start-end.exp: New file.
	* gdb.dwarf2/loclists-start-end.c: New file.