archivers/p7zip: fix build on powerpc64

lld 10 reports the following error:
ld: error: relocation R_PPC64_TOC16_LO cannot be used against symbol MatchFinder_GetNumAvailableBytes; recompile with -fPIC

defined in LzFind.o
referenced by LzFind.c

LzFind.o:(MatchFinder_CreateVTable)