[gdb/testsuite] Fix gdb.ada/catch_ex_std.exp gnatlink FAIL

[gdb/testsuite] Fix gdb.ada/catch_ex_std.exp gnatlink FAIL

When running test-case gdb.ada/catch_ex.exp using system gnatmake, gnatmake is
invoked like this:
...
Executing on host: \
  gnatmake foo.adb -gnata -f -Isrc/gdb/testsuite/gdb.ada/catch_ex -g -lm \
    -o outputs/gdb.ada/catch_ex/foo
...

When I try to use a more recent gnatmake, by mocking up a combined build:
...
$ ls -la build/gcc/
lrwxrwxrwx  gfortran -> /usr/bin/gfortran-10
lrwxrwxrwx  gnatbind -> /usr/bin/gnatbind-10
lrwxrwxrwx  gnatlink -> /usr/bin/gnatlink-10
lrwxrwxrwx  gnatmake -> /usr/bin/gnatmake-10
lrwxrwxrwx  xg++ -> /usr/bin/g++-10
lrwxrwxrwx  xgcc -> /usr/bin/gcc-10
...
gnatmake is invoked like this:
...
Executing on host: \
  /data/gdb_versions/devel/build/gcc/gnatmake \
    -I/data/gdb_versions/devel/build/gcc/ada/rts \
    --GCC=/data/gdb_versions/devel/build/gcc/xgcc \
    --GNATBIND=/data/gdb_versions/devel/build/gcc/gnatbind \
    --GNATLINK=/data/gdb_versions/devel/build/gcc/gnatlink \
    -cargs -B/data/gdb_versions/devel/build/gcc \
    -largs --GCC=/data/gdb_versions/devel/build/gcc/xgcc \
    -B/data/gdb_versions/devel/build/gcc \
    -margs foo.adb -gnata -f -Isrc/gdb/testsuite/gdb.ada/catch_ex -g -lm \
    -o outputs/gdb.ada/catch_ex/foo
...

This is set up by this bit in find_gnatmake in
/usr/share/dejagnu/libgloss.exp:
...
    if {![is_remote host]} {
        set file [lookfor_file $tool_root_dir gnatmake]
        if { $file == "" } {
            set file [lookfor_file $tool_root_dir gcc/gnatmake]
        }
        if { $file != "" } {
            set root [file dirname $file]
            set CC "$file -I$root/ada/rts --GCC=$root/xgcc \
	      --GNATBIND=$root/gnatbind --GNATLINK=$root/gnatlink \
	      -cargs -B$root \
	      -largs --GCC=$root/xgcc -B$root -margs"
        } else {
...

However, when running test-case gdb.ada/catch_ex_std.exp using the mockup
combined build, we get:
...
Executing on host: \
  /data/gdb_versions/devel/build/gcc/gnatlink foo \
    -Wl,-rpath,\$ORIGIN -Wl,-lsome_package

b~foo.adb:26:79: "SS_Stack" not declared in "Secondary_Stack"^M
b~foo.adb:26:89: incorrect constraint for this kind of type^M
b~foo.adb:121:56: "Runtime_Default_Sec_Stack_Size" not declared in "Parameters"^M
FAIL: gdb.ada/catch_ex_std.exp: gnatlink foo
...

The problem is caused by the fact that the test uses gnatlink directly
rather than using gnatmake.  The invoked gnatlink (which is gnatlink-10) calls
gcc-7, which are incompatible (see gcc PR86211).  This problem doesn't occur
with gnatmake because there the gcc to use is passed as an argument to
gnatlink.

Fix this by adding the -largs bit from find_gnatmake in find_ada_tool, for the
case that $tool == gnatlink.

Tested on x86_64-linux, with system gcc, and gcc-10.

gdb/testsuite/ChangeLog:

2020-04-13  Tom de Vries  <tdevries@suse.de>

	* lib/ada.exp (find_ada_tool): Pass --GCC and -B to gnatlink, similar
	to what find_gnatmake does.