[gdb/go] Handle v3 go_0 mangled prefix

With gcc-10 we have:
...
(gdb) break package2.Foo^M
Breakpoint 2 at 0x402563: file package2.go, line 5.^M
(gdb) PASS: gdb.go/package.exp: setting breakpoint 1
...
but with gcc-11:
...
gdb) break package2.Foo^M
Function "package2.Foo" not defined.^M
Make breakpoint pending on future shared library load? (y or [n]) n^M
(gdb) FAIL: gdb.go/package.exp: gdb_breakpoint: set breakpoint at package2.Foo
...

In the gcc-10 case, though the exec contains dwarf, it's not used to set the
breakpoint (which is an independent problem, filed as PR go/30941), instead
the minimal symbol information is used.

The minimal symbol information changed between gcc-10 and gcc-11:
...
$ nm a.out.10 | grep Foo
000000000040370d T go.package2.Foo
0000000000404e50 R go.package2.Foo..f
$ nm a.out.11 | grep Foo
0000000000403857 T go_0package2.Foo
0000000000405030 R go_0package2.Foo..f
...

A new v3 mangling scheme was used.  The mangling schemes define a separator
character and mangling character:
- for v2, dot is used both as separator character and mangling character, and
- for v3, dot is used as separator character and underscore as mangling
  character.

For more details, see [1] and [2].

In v3, "_0" demangles to ".". [ See gcc commit a01dda3c23b ("compiler, libgo:
change mangling scheme"), function Special_char_code::Special_char_code. ]

Handle the new go_0 prefix in unpack_mangled_go_symbol, which fixes the
test-case.

Note that this doesn't fix this regression:
...
$ gccgo-10 package2.go -c -g0
$ gccgo-10 package1.go package2.o -g0
$ gdb -q -batch a.out -ex "break go.package2.Foo"
Breakpoint 1 at 0x40370d
$ gccgo-11 package2.go -c -g0
$ gccgo-11 package1.go package2.o -g0
$ gdb -q -batch a.out -ex "break go.package2.Foo"
Function "go.package2.Foo" not defined.
...

With gcc-10, we set a breakpoint on the mangled minimal symbol.  That
one has simply changed for gcc-11, so it's equivalent to using:
...
$ gdb -q -batch a.out -ex "break go_0package2.Foo"
Breakpoint 1 at 0x403857
...
which does work.

Tested on x86_64-linux:
- openSUSE Leap 15.4, using gccgo-7,
- openSUSE Tumbleweed, using gccgo-13.

PR go/27238
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27238

[1] https://go-review.googlesource.com/c/gofrontend/+/271726
[2] https://github.com/golang/go/issues/41862#issuecomment-707244103

gdb/go-lang.c

@@ -233,16 +233,28 @@ unpack_mangled_go_symbol (const char *mangled_name,
      libgo_.*: used by gccgo's runtime
 
      Thus we don't support -fgo-prefix (except as used by the runtime).  */
-  if (!startswith (mangled_name, "go.")
-      && !startswith (mangled_name, "libgo_"))
+  bool v3;
+  if (startswith (mangled_name, "go_0"))
+    /* V3 mangling detected, see
+       https://go-review.googlesource.com/c/gofrontend/+/271726 .  */
+    v3 = true;
+  else if (startswith (mangled_name, "go.")
+	   || startswith (mangled_name, "libgo_"))
+    v3 = false;
+  else
     return NULL;
 
   /* Quick check for whether a search may be fruitful.  */
   /* Ignore anything with @plt, etc. in it.  */
   if (strchr (mangled_name, '@') != NULL)
     return NULL;
+
   /* It must have at least two dots.  */
-  first_dot = strchr (mangled_name, '.');
+  if (v3)
+    first_dot = strchr (mangled_name, '0');
+  else
+    first_dot = strchr (mangled_name, '.');
+
   if (first_dot == NULL)
     return NULL;
   /* Treat "foo.bar" as unmangled.  It can collide with lots of other
@@ -263,6 +275,18 @@ unpack_mangled_go_symbol (const char *mangled_name,
   gdb::unique_xmalloc_ptr<char> result = make_unique_xstrdup (mangled_name);
   buf = result.get ();
 
+  if (v3)
+    {
+      /* Replace "go_0" with "\0go.".  */
+      buf[0] = '\0';
+      buf[1] = 'g';
+      buf[2] = 'o';
+      buf[3] = '.';
+
+      /* Skip the '\0'.  */
+      buf++;
+    }
+
   /* Search backwards looking for "N<digit(s)>".  */
   p = buf + len;
   saw_digit = method_type = NULL;