Skip testing argv[0] on target argv[0] isn't available

I see the following two fails on arm-none-eabi target, because argv[0]
isn't available.

print argv[0]^M
$1 = 0x1f78 "/dev/null"^M
(gdb) FAIL: gdb.base/argv0-symlink.exp: kept file symbolic link name

print argv[0]^M
$1 = 0x1f78 "/dev/null"^M
(gdb) FAIL: gdb.base/argv0-symlink.exp: kept directory symbolic link name

My first thought is to check [target_info exists noargs], and skip the
test if it returns true.  However, noargs is set in gdbserver board
files, so argv0-symlink.exp will be skipped on gdbserver board file.
The change is too aggressive.

When the program is running with gdbserver, argv[1] to argv[N] aren't
available, but argv[0] is.  Fortunately, argv0-symlink.exp only
requires argv[0].  argv0-symlink.exp can be run with gdbserver board
file, as what we do now.

What we need to check is whether argv[0] is available, so I add a new
proc gdb_has_argv0 to do so by starting a program, and check
argc/argv[0] to see whether argv[0] is available.

Dan fixed the similar problem by checking noargs, which is too strong.
https://sourceware.org/ml/gdb-patches/2010-02/msg00398.html as a
result, the test is skipped on gdbserver.  This patch fixed it too.

gdb/testsuite:

2014-10-18  Yao Qi  <yao@codesourcery.com>

	* gdb.base/argv0-symlink.exp: Check argv[0] value if
	gdb_has_argv0 return true.
	* gdb.guile/scm-value.exp (test_value_in_inferior): Don't
	check [target_info exists noargs], check [gdb_has_argv0]
	instead.
	* gdb.python/py-value.exp (test_value_in_inferior): Likewise.
	* lib/gdb.exp (gdb_has_argv0, gdb_has_argv0_1): New
	procedures.

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