gdb/testsuite: gdb.base/gcorebg.exp against installed binaries

gdb/testsuite: gdb.base/gcorebg.exp against installed binaries

It is possible to run GDB's testsuite against installed binaries rather
than the one from the build tree.  For example, one could do:

   $ make check-gdb RUNTESTFLAGS=GDB=/usr/bin/gdb

Running the testsuite this way causes error for gdb.base/gcorebg.exp:

    Running [...]/gdb/testsuite/gdb.base/gcorebg.exp ...
    FAIL: gdb.base/gcorebg.exp: detached=detached: Spawned gcore finished
    FAIL: gdb.base/gcorebg.exp: detached=detached: Core file generated by gcore
    FAIL: gdb.base/gcorebg.exp: detached=standard: Spawned gcore finished
    FAIL: gdb.base/gcorebg.exp: detached=standard: Core file generated by gcore

This is due to 2 problems.
First, when running this way, the $GDB_DATA_DIRECTORY is not set (on
purpose) as the installed GDB does not need to be specified where to
find it.  See this section in gdb/testsuite/lib/gdb.exp:

    if ![info exists GDB] {
        [....]
    } else {
        # If the user specifies GDB on the command line, and doesn't
        # specify GDB_DATA_DIRECTORY, then assume we're testing an
        # installed GDB, and let it use its own configured data directory.
        if ![info exists GDB_DATA_DIRECTORY] {
            set GDB_DATA_DIRECTORY ""
        }
    }

The testbg.exp file always assumes a non-empty GDB_DATA_DIRECTORY.  As a
consequence, when calling the gcorebg binary with an empty argument
(i.e. missing argument), the program fails:

    gcorebg: [...]/gdb/testsuite/gdb.base/gcorebg.c:42: main: Assertion `argc == 5' failed.
    FAIL: gdb.base/gcorebg.exp: detached=standard: Spawned gcore finished

This patch does adjust the gcorebg.c and gcorebg.exp files to allow not
specifying the data-directory.

The other issue is that the testsuite assumes that the `gcore` to test
is always the one from the build tree.  However, if someone is testing
an installed binary by setting GDB, I think that person would expect to
test the `gcore` script next to the binary that was specified (unless
GCORE is specified to explicitly specified).  This patch does that
adjustment as well.  To that end, it needs to move the block setting
GCORE after the block setting GDB.

Change-Id: I070e039903c0b5afeac357d8fac7d710ff6697b9
Approved-By: Tom Tromey <tom@tromey.com>