[gdb/testsuite] Avoid using .eh_frame in gdb.base/unwind-on-each-insn.exp

[gdb/testsuite] Avoid using .eh_frame in gdb.base/unwind-on-each-insn.exp

One purpose of the gdb.base/unwind-on-each-insn.exp test-case is to test the
architecture-specific unwinders on foo, so unwind-on-each-insn-foo.c is
compiled with nodebug, to prevent the dwarf unwinders from taking effect.

For for instance gcc x86_64 though, -fasynchronous-unwind-tables is enabled by
default, generating an .eh_frame section contribution which might enable the
dwarf unwinders and bypass the architecture-specific unwinders.

Currently, that happens to be not the case due to the current implementation
of epilogue_unwind_valid, which assumes that in absence of debug info proving
that the compiler is gcc >= 4.5.0, the .eh_frame contribution is invalid.

That may change though, see PR30028, in which case
gdb.base/unwind-on-each-insn.exp stops being a regression test for commit
49d7cd733a7 ("Change calculation of frame_id by amd64 epilogue unwinder").

Fix this by making sure that we don't use .eh_frame info regardless of
epilogue_unwind_valid, simply by not generating it using
-fno-asynchronous-unwind-tables.

Tested on x86_64-linux, target boards unix/{-m64,-m32}, using compilers
gcc 7.5.0 and clang 13.0.1.