package/gdb: fix gnulib issue with musl and uClibc toolchains (finally)

package/gdb: fix gnulib issue with musl and uClibc toolchains (finally)

The previous fix [1] reinroduced the issue fixed by [2].
So keep gl_cv_func_gettimeofday_clobber=no in GDB_CONF_ENV
and GDB_MAKE_ENV.

Fixes:
http://autobuild.buildroot.net/results/ec6/ec60d436bd34a4f37b664e4124d7f0c96e90a1be

[1] faf38b78aee613a732f96d5022a2894efd024a25
[2] 560334bb2a46a77081a3944d092df7732dfca432

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>