[gdb/testsuite] Fix clean_restart <absolute filename> in gdb.arch/amd64*.exp

Fix clean_restart <absolute filename> in gdb.arch/amd64*.exp.

Tested on x86_64-linux, with kernel version 6.16.3.

The only unsupported test-case is gdb.arch/amd64-lam.exp, which isn't changed
by this commit.