[gdb/testsuite] Fix clean_restart <absolute filename> in gdb.disasm

Fix clean_restart <absolute filename> in the test-cases in gdb.disasm.

The changed test-cases:
- gdb.disasm/am33.exp
- gdb.disasm/hppa.exp
- gdb.disasm/mn10300.exp
- gdb.disasm/sh3.exp
are unsupported for me, but the changes are trivial.