brltty: use gcc instead of ld to link shared objects

brltty: use gcc instead of ld to link shared objects

Bump to version 5.6 has reintroduced the issue fixed by patch 854093 on
version 5.5.

Indeed, third patch was merged upstream but it was then reverted:
https://github.com/brltty/brltty/commit/9e7d62c869d3c1cbe12dda8b0291a4692c193416

Moreover, since commit 3a2e3f6fa5ef0a210ffeba5ed05c79965d0cc3c7, MKOBJ
was renamed into MKREL. So, patch again brltty, overwrite
MKREL to use "gcc -shared -o" instead of "ld -r -o" if gcc is available
(as suggested by Arnout: https://patchwork.ozlabs.org/patch/972614).

Fixes:
 - http://autobuild.buildroot.org/results/31f682838b3d3b2c7103b5c51f2aba0b89d4f630

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>