[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
49d7cd733a ("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.

gdb/testsuite/gdb.base/unwind-on-each-insn.exp

@@ -25,9 +25,19 @@
 
 standard_testfile .c -foo.c
 
+set debug_flags {debug}
+set nodebug_flags {nodebug}
+
+# Make sure that we don't use .eh_frame info, by not generating it,
+# using -fno-asynchronous-unwind-tables, if supported.
+if { [gdb_can_simple_compile fno-asynchronous-unwind-tables \
+	  { void foo () { } } object -fno-asynchronous-unwind-tables] } {
+    lappend nodebug_flags additional_flags=-fno-asynchronous-unwind-tables
+}
+
 if {[prepare_for_testing_full "failed to prepare" \
-	 [list ${testfile} debug \
-	      $srcfile {debug} $srcfile2 {nodebug}]]} {
+	 [list ${testfile} $debug_flags \
+	      $srcfile $debug_flags $srcfile2 $nodebug_flags]]} {
     return -1
 }