Use dlsym to check if libdl is needed for plugin

config/plugins.m4 has

 if test "$plugins" = "yes"; then
    AC_SEARCH_LIBS([dlopen], [dl])
  fi

Plugin uses dlsym, but libasan.so only intercepts dlopen, not dlsym:

[hjl@gnu-tools-1 binutils-text]$ nm -D /lib64/libasan.so.4| grep " dl"
0000000000038580 W dlclose
                 U dl_iterate_phdr
000000000004dc50 W dlopen
                 U dlsym
                 U dlvsym
[hjl@gnu-tools-1 binutils-text]$

Testing dlopen for libdl leads to false negative when -fsanitize=address
is used.  It results in link failure:

../bfd/.libs/libbfd.a(plugin.o): undefined reference to symbol 'dlsym@@GLIBC_2.16'

dlsym should be used to check if libdl is needed for plugin.

	PR gas/22318
	* plugins.m4 (AC_PLUGINS): Use dlsym to check if libdl is needed.

From-SVN: r259140