[gdb/testsuite] Fix "unable to find usable gdb" error with cc-with-tweaks.exp

[gdb/testsuite] Fix "unable to find usable gdb" error with cc-with-tweaks.exp

When running fullpath-expand.exp with target_board=dwarf4-gdb-index, we run
into:
...
$ make check-gdb RUNTESTFLAGS="--target_board=dwarf4-gdb-index fullpath-expand.exp"
Running src/gdb/testsuite/gdb.base/fullpath-expand.exp ...
gdb compile failed, cc-with-tweaks.sh: unable to find usable gdb

                === gdb Summary ===

nr of untested testcases         1
...
The same happens with fullname.exp.

The dwarf4-gdb-index.exp board file includes cc-with-tweaks.exp, which uses
cc-with-tweaks.sh, which calls gdb-add-index.sh.

The gdb-add-index.sh script uses a gdb executable, defaulting to gdb:
...
GDB=${GDB:=gdb}
...

The cc-with-tweaks.sh script tries to ensure that the build gdb executable is
used by gdb-add-index.sh:
...
if [ -z "$GDB" ]
then
    if [ -f ./gdb ]
    then
	GDB="./gdb -data-directory data-directory"
    elif [ -f ../gdb ]
    then
	GDB="../gdb -data-directory ../data-directory"
    elif [ -f ../../gdb ]
    then
	GDB="../../gdb -data-directory ../../data-directory"
    else
	echo "$myname: unable to find usable gdb" >&2
	exit 1
    fi
fi
...
So, if the current directory is build/gdb/testsuite, then a gdb executable
build/gdb/testsuite/../gdb will be used.

However, in the case of fullpath-expand.exp the test cd's into the sources:
...
set saved_pwd [pwd]
cd $srcdir
set err [gdb_compile "${subdir}/${srcfile} ${subdir}/${srcfile2}" $binfile \
         executable {debug}]
cd $saved_pwd
...
and cc-with-tweaks.sh generates the "unable to find usable gdb" error.

The same error occurs if we use --target_board=cc-with-dwz instead (only in
this case we actually don't need gdb, we just need the GDB variable to be set
in cc-with-tweaks.sh, which arguably is a bug in cc-with-tweaks.sh).

Fix both errors in cc-with-tweaks.exp by generating a gdb script gdb.sh using
$GDB, $GDBFLAGS and $INTERNAL_GDBFLAGS and passing this script to
cc-with-tweaks.sh by setting env(GDB).

Tested on x86_64-linux for gdb.base.

gdb/testsuite/ChangeLog:

2019-05-01  Tom de Vries  <tdevries@suse.de>

	* boards/cc-with-tweaks.exp: Generate gdb.sh, and pass it in env(GDB).