boost: build context for mips64

It turns out there's upstream support for it. A small patch is needed
to fix softfloat support.

Also added patch to fix boost-fiber on octeon+. Failure happens because
the platform is based on an old MIPSr2 standard that lacks the pause
instruction.

It also turns out that MIPS64 builds are done with the wrong ABI.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

libs/boost/patches/020-mips64-fiber.patch

@@ -0,0 +1,11 @@
+--- a/boost/fiber/detail/cpu_relax.hpp
++++ b/boost/fiber/detail/cpu_relax.hpp
+@@ -47,7 +47,7 @@ namespace detail {
+ # else
+ #  define cpu_relax() asm volatile ("nop" ::: "memory");
+ # endif
+-#elif BOOST_ARCH_MIPS && (__mips_isa_rev > 1)
++#elif BOOST_ARCH_MIPS && (__mips_isa_rev > 1) && !defined(_MIPS_ARCH_OCTEONP)
+ # define cpu_relax() asm volatile ("pause" ::: "memory");
+ #elif BOOST_ARCH_PPC
+ // http://code.metager.de/source/xref/gnu/glibc/sysdeps/powerpc/sys/platform/ppc.h