[gdb/testsuite] Fix gdb.go/methods.exp

[gdb/testsuite] Fix gdb.go/methods.exp

With gccgo-6/7, we have:
...
FAIL: gdb.go/methods.exp: setting breakpoint at main.T.Foo
XFAIL: gdb.go/methods.exp: going to first breakpoint \
  (the program exited)
FAIL: gdb.go/methods.exp: setting breakpoint at (*main.T).Bar
XFAIL: gdb.go/methods.exp: going to second breakpoint \
  (the program is no longer running)
...

And with gccgo-8/9/10, we have:
...
PASS: gdb.go/methods.exp: setting breakpoint 1
XFAIL: gdb.go/methods.exp: going to first breakpoint
FAIL: gdb.go/methods.exp: setting breakpoint at (*main.T).Bar
XFAIL: gdb.go/methods.exp: going to second breakpoint \
  (the program exited)
...

The first test passes and fails with different messages:
...
FAIL: gdb.go/methods.exp: setting breakpoint at main.T.Foo
...
or:
...
PASS: gdb.go/methods.exp: setting breakpoint 1
...
Fix this by removing the explicit pass call and using the message argument for
gdb_breakpoint, for both breakpoint locations.

The setup of the xfails is non-specific:
...
setup_xfail "*-*-*" ;# mangling issues IIRC
...
so let's start with removing these.

The first FAIL with gccgo-6:
...
FAIL: gdb.go/methods.exp: setting breakpoint at main.T.Foo
...
is due an incorrect DW_AT_name attribute:
...
    #    <554>   DW_AT_name        : main.Foo.N6_main.T
...
Fix this by recognizing the incorrect attribute, and xfailing the test.

Furthermore, if setting the breakpoint fails, there's not much point in trying
to continue to the breakpoint:
...
FAIL: gdb.go/methods.exp: setting breakpoint at main.T.Foo
FAIL: gdb.go/methods.exp: going to first breakpoint (the program exited)
...
Fix this by skipping the second test if the first one fails, also for the
second breakpoint.

With gccgo-10, we manage to set the first breakpoint, but continuing to
breakpoint test fails:
...
PASS: gdb.go/methods.exp: setting breakpoint 1
FAIL: gdb.go/methods.exp: going to first breakpoint
...
This is due to an incorrect regexp, requiring a colon in front of the
breakpoint location.  Fix this for both breakpoints.

Setting the second breakpoint fails:
...
FAIL: gdb.go/methods.exp: setting breakpoint at (*main.T).Bar
...
presumably because the breakpoint location "(*main.T).Bar" does not follow the
naming convention explained at https://golang.org/doc/gdb#Naming.  Fix this by
updating the breakpoint location to "main.(*T).Bar".

Still this test fails, for gccgo-6/7 because of an incorrect DW_AT_name
attribute:
...
    # <529>   DW_AT_name        : main.Bar.pN6_main.T
...
and for gccgo-8/9/10 because of incorrect DW_AT_name/DW_AT_linkage_name
attributes (filed as gcc PR93866):
...
    #    <6e5>   DW_AT_name        : main.Bar..1main.T
    #    <6ec>   DW_AT_linkage_name: main.T.Bar
..
Add xfails for both of these.

All in all, now we have with gccgo-6/7:
...
XFAIL: gdb.go/methods.exp: setting breakpoint at main.T.Foo
XFAIL: gdb.go/methods.exp: setting breakpoint at main.(*T).Bar
...
and with gccgo-8/9/10, we have:
...
PASS: gdb.go/methods.exp: setting breakpoint at main.T.Foo
PASS: gdb.go/methods.exp: going to first breakpoint
XFAIL: gdb.go/methods.exp: setting breakpoint at main.(*T).Bar
...

Tested on x86_64-linux with gccgo-6/7/8/9/10.

gdb/testsuite/ChangeLog:

2020-02-21  Tom de Vries  <tdevries@suse.de>

	PR go/18926
	* lib/gdb.exp (bp_location2/bp_location2_regexp): Fix.
	Remove blanket xfails.  Use message argument for gdb_breakpoint.
	Make continuing to breakpoint test conditional on setting breakpoint.
	Fix continuing to breakpoint regexp.  Add xfails for gccgo-6/7
	DW_AT_name attribute.  Add xfail for GCC PR93866.