package/p7zip: needs __sync_*_4 intrisics

package/p7zip: needs __sync_*_4 intrisics

p7zip needs __sync_*_4 intrisics since its bump to version 17.04 in
commit 9332ab86f8953a8710b9377d3fba46c986610254 and
https://github.com/jinfeihan57/p7zip/commit/b0bf49e3e14ef291281b6c318c75db96c03910bc
https://github.com/jinfeihan57/p7zip/commit/6175cdaf3e762cc9e03c8af00e76ffe60d3e0844

Fixes:
 - http://autobuild.buildroot.org/results/4e1a920d09528b87803cc2e7245a470c85548946

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