Fix in-tree, parallel running of Ada tests

Fix in-tree, parallel running of Ada tests

While testing the following patch,

  [PATCH] Always organize test artifacts in a directory hierarchy
  https://sourceware.org/ml/gdb-patches/2016-01/msg00133.html

I noticed that it broke Ada testing.  This lead me to think that
parallel testing when building in-tree didn't work previously in Ada.
It is confirmed by this test:

$ make check TESTS="gdb.ada/fun_addr.exp" -j 2
...
Running ./gdb.ada/fun_addr.exp ...
FAIL: gdb.ada/fun_addr.exp: compilation foo.adb
...

This patch fixes in-tree parallel testing for Ada, and consequently
serial and parallel testing when the aforementioned patch is applied.

The problem originates from the fact that Ada support code cd's to the
builddir before compiling.  In itself it's not a problem, it allows to
place intermediate auto-generated files in that directory.  The Ada
compilation refers to the source file, which is in another directory,
only by its base name (e.g. foo.adb).  In serial mode, that worked
because builddir was the same as the source directory (e.g.
gdb.ada/fun_addr/).  In an out-of-tree build, it works because the
source directory is added as an include directory (note: this is not the
same $srcdir as autoconf's):

  set srcdir [file dirname $source]
  additional_flags=-I$srcdir

which becomes:

  additional_flags=-I/home/emaisin/build/binutils-gdb/gdb/testsuite/gdb.ada/fun_addr

However, when building in-tree, srcdir is relative: ./gdb.ada/fun_addr.
When using parallel or always-in-outputs-directory mode, we are cd'ed in
the outputs directory.  So -I$srcdir is relative to the current
directory, which is wrong.

To fix it, I made the TCL variable srcdir (set in site.exp, from which
everything else is derived) always absolute.  It is done by assigning
autoconf's abs_srcdir instead of autoconf's srcdir.  This way -I$srcdir
will always be good, regardless of where we cd'ed to.  A small apparent
change is that when running tests, DejaGnu will say:

  Running /tmp/binutils-gdb/gdb/testsuite/gdb.ada/fun_addr.exp ...

instead of

  Running ./gdb.ada/fun_addr.exp ...

I hope it's not too much of an annoyance.  I think that it should make
the testsuite a tiny bit more robust against other bugs of the same
class.

Regtested in & out of tree, only with native target.

gdb/testsuite/ChangeLog:

	* Makefile.in (abs_srcdir): Assign @abs_srcdir@.
	(site.exp): Assign abs_srcdir to tcl's srcdir.