gdb: Update i386_analyze_prologue to skip endbr32

gdb: Update i386_analyze_prologue to skip endbr32

With -m32 -fcf-protection, GCC generates an `endbr32` instruction at the
function entry:

[hjl@gnu-cfl-2 gdb]$ cat /tmp/x.c
int
main(void)
{
  return 0;
}
[hjl@gnu-cfl-2 gdb]$ gcc -g -fcf-protection /tmp/x.c -m32
(gdb) b main
Breakpoint 1 at 0x8049176: file /tmp/x.c, line 3.
(gdb) r
Breakpoint 1, main () at /tmp/x.c:3
3	{
(gdb) disass
Dump of assembler code for function main:
=> 0x08049176 <+0>:	endbr32
   0x0804917a <+4>:	push   %ebp
   0x0804917b <+5>:	mov    %esp,%ebp
   0x0804917d <+7>:	mov    $0x0,%eax
   0x08049182 <+12>:	pop    %ebp
   0x08049183 <+13>:	ret
End of assembler dump.
(gdb)

Update i386_analyze_prologue to skip `endbr32`:

(gdb) b main
Breakpoint 1 at 0x804917d: file /tmp/x.c, line 4.
(gdb) r
Breakpoint 1, main () at /tmp/x.c:4
4	  return 0;
(gdb) disass
Dump of assembler code for function main:
   0x08049176 <+0>:	endbr32
   0x0804917a <+4>:	push   %ebp
   0x0804917b <+5>:	mov    %esp,%ebp
=> 0x0804917d <+7>:	mov    $0x0,%eax
   0x08049182 <+12>:	pop    %ebp
   0x08049183 <+13>:	ret
End of assembler dump.
(gdb)

Tested with

$ make check RUNTESTFLAGS="--target_board='unix{-m32,}' i386-prologue-skip-cf-protection.exp"

on Fedora 32/x86-64.

2020-0X-YY  Victor Collod  <vcollod@nvidia.com>

gdb/ChangeLog:

	PR gdb/26635
	* i386-tdep.c (i386_skip_endbr): Add a helper function to skip endbr.
	(i386_analyze_prologue): Call i386_skip_endbr.

gdb/testsuite/ChangeLog:

	PR gdb/26635
	* gdb.arch/amd64-prologue-skip-cf-protection.exp: Make the test
	compatible with i386, and move it to...
	* gdb.arch/i386-prologue-skip-cf-protection.exp: ... here.
	* gdb.arch/amd64-prologue-skip-cf-protection.c: Move to...
	* gdb.arch/i386-prologue-skip-cf-protection.c: ... here.