While squid uses the proper autoconf logic to check if the -latomic is required
for 64-bit atomics, its conftest.cpp fails to compile without -fpermissive thus
yielding wrong result on 32-bit MIPS (which, until R6, lacks 64-bit atomics).

Do not bother with configure{.ac} patches (too bulky), do not add -fpermissive
to CXXFLAGS just to fix the conftest.cpp compilation (may affect too much) but
simply pass -latomic preemptibly on mips{el}.

As usual (no previous mips* apks exist, other architectures are unaffected),
keep pkgrel intact.

This commit should be removed when (if) upstream (or, really, autoconf) fixes
the problem.