[gdb/testsuite] Simplify gdb.base/unwind-on-each-insn.exp.tcl

Recent commit 1d98e564 ("[gdb/testsuite] Add
gdb.base/unwind-on-each-insn-{amd64,i386}.exp") broke commit eb015bf8
("[gdb/testsuite] Avoid using .eh_frame in gdb.base/unwind-on-each-insn.exp"),
in the sense that gdb.base/unwind-on-each-insn.exp no longer uses
-fno-asynchronous-unwind-tables, due to trying to concatenate two lists using:
...
    lappend srcfile2_flags $nodebug_flags
...
which should instead be:
...
    lappend srcfile2_flags {*}$nodebug_flags
...

Fix this by simplifying gdb.base/unwind-on-each-insn.exp.tcl, completely
leaving the responsibility to set srcfile_flags and srcfile2_flags to each
includer.

Tested on x86_64-linux.