[gdb/testsuite] Fix gdb.opt/clobbered-registers-O2.exp with clang

When running test-case gdb.opt/clobbered-registers-O2.exp with clang 12.0.1, I
get:
...
(gdb) run ^M
Starting program: clobbered-registers-O2 ^M
^M
Program received signal SIGSEGV, Segmentation fault.^M
gen_movsd (operand0=<optimized out>, operand1=<optimized out>) at \
  clobbered-registers-O2.c:31^M
31              return *start_sequence(operand0, operand1);^M
(gdb) FAIL: gdb.opt/clobbered-registers-O2.exp: runto: run to start_sequence
...

The problem is that the breakpoint in start_sequence doesn't trigger, because:
- the call to start_sequence in gen_movsd is optimized away, despite the
  __attribute__((noinline)), so the actual function start_sequence doesn't get
  called, and
- the debug info doesn't contain inlined function info, so there's only one
  breakpoint location.

Adding noclone and noipa alongside the noinline attribute doesn't fix this.

Adding the clang-specific attribute optnone in start_sequence does, but since
it inhibits all optimization, that's not a preferred solution in a gdb.opt
test-case, and it would work only for clang and not other compilers that
possibly have the same issue.

Fix this by moving functions start_sequence and gen_movsd into their own
files, as a way of trying harder to enforce noinline/noipa/noclone.

Tested on x86_64-linux.

gdb/testsuite/gdb.opt/clobbered-registers-O2-2.c

@@ -0,0 +1,24 @@
+/* This file is part of GDB, the GNU debugger.
+
+   Copyright 2007-2022 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+extern unsigned *start_sequence (unsigned *x, unsigned *y);
+
+unsigned
+gen_movsd (unsigned *operand0, unsigned *operand1)
+{
+  return *start_sequence (operand0, operand1);
+}

gdb/testsuite/gdb.opt/clobbered-registers-O2-3.c

@@ -0,0 +1,22 @@
+/* This file is part of GDB, the GNU debugger.
+
+   Copyright 2007-2022 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+unsigned *
+start_sequence (unsigned *x, unsigned *y)
+{
+  return (unsigned *)0xdeadbeef;
+}

gdb/testsuite/gdb.opt/clobbered-registers-O2.c

@@ -15,21 +15,7 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
-#ifndef __GNUC__
-#define __attribute__(x)
-#endif
-
-unsigned * __attribute__((noinline))
-start_sequence (unsigned * x, unsigned * y)
-{
-	return (unsigned *)0xdeadbeef;
-};
-
-unsigned __attribute__((noinline))
-gen_movsd (unsigned * operand0, unsigned * operand1)
-{
-	return *start_sequence(operand0, operand1);
-}
+extern unsigned gen_movsd (unsigned *operand0, unsigned *operand1);
 
 int main(void)
 {

gdb/testsuite/gdb.opt/clobbered-registers-O2.exp

@@ -18,7 +18,7 @@
 # Test displaying call clobbered registers in optimized binaries.
 # GDB should not show incorrect values.
 
-standard_testfile
+standard_testfile .c -2.c -3.c
 
 # What compiler are we using?
 #
@@ -26,7 +26,8 @@ if [get_compiler_info] {
     return -1
 }
 
-if {[prepare_for_testing "failed to prepare" $testfile $srcfile \
+if {[prepare_for_testing "failed to prepare" $testfile \
+	 [list $srcfile $srcfile2 $srcfile3] \
 	 {debug optimize=-O2 nowarnings}]} {
     return -1
 }