package/gdb: add version 8.1

package/gdb: add version 8.1

https://sourceware.org/ml/gdb-announce/2018/msg00001.html

gdb 8.1 has a new optional dependency on mpfr, which according to the
NEWS file:

   GDB now uses the GNU MPFR library, if available, to emulate target
   floating-point arithmetic during expression evaluation when the
   target uses different floating-point formats than the host.  At
   least version 3.1 of GNU MPFR is required.

So for the target gdb, this is unnecessary, and therefore we
forcefully disable mpfr support by passing --without-mpfr.

For the host gdb, it would potentially be useful, but since it's a new
feature that isn't essential, we for now keep it disabled as well. An
option may be added later if needed.

To avoid gnulib issues with uClibc/musl, we need to pass additional
configure/make variables, like is done in OpenEmbedded [1].

[1] http://git.openembedded.org/openembedded-core/tree/meta/recipes-devtools/gdb/gdb_8.1.bb?id=02664a8dcd1ebb6cd77248e1b97a78390ea06033#n9

We re-add the same patches as the ones present for gdb 8.0.1, except
0004-nat-linux-ptrace.c-add-missing-gdb_byte-cast.patch, which was
merged upstream.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
[Thomas: change mpfr handling.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>