gdb: pass core file to gdbarch_core_xfer_siginfo

Another patch that aims to remove 'current_program_space->core_bfd ()'
from GDB.  This time I'm passing the core file BFD as an argument to
the gdbarch method gdbarch_core_xfer_siginfo.

In corelow.c the core file is being passed, this does introduce a new
instance of 'current_program_space->core_bfd ()', but this is OK.  My
long term plan is to move the core bfd into core_target, in which case
the call to gdbarch_core_xfer_siginfo will have access to the core bfd
as a member variable.

For now though, this patch moves the accesses via global state up the
call stack, and consolidates the two calls from fbsd-tdep.c and
linux-tdep.c into the one call in corelow.c.

There should be no user visible changes after this commit.

Approved-By: Simon Marchi <simon.marchi@efficios.com>

gdb/gdbarch_components.py

@@ -1777,12 +1777,17 @@ How the core target extracts the name of a thread from a core file.
 Method(
     comment="""
 Read offset OFFSET of TARGET_OBJECT_SIGNAL_INFO signal information
-from core file into buffer READBUF with length LEN.  Return the number
+from core file CBFD into buffer READBUF with length LEN.  Return the number
 of bytes read (zero indicates EOF, a negative value indicates failure).
 """,
     type="LONGEST",
     name="core_xfer_siginfo",
-    params=[("gdb_byte *", "readbuf"), ("ULONGEST", "offset"), ("ULONGEST", "len")],
+    params=[
+        ("struct bfd &", "cbfd"),
+        ("gdb_byte *", "readbuf"),
+        ("ULONGEST", "offset"),
+        ("ULONGEST", "len")
+    ],
     predicate=True,
 )