libjuice: fix SHA-1 on big-endian targets

libjuice: fix SHA-1 on big-endian targets

libjuice's vendored picohash guards the final byte-order swap in
_picohash_sha1_final with SHA_BIG_ENDIAN, which nothing defines, while
endianness is detected into _PICOHASH_BIG_ENDIAN. On big-endian targets
the swap runs when it must not and every SHA-1 digest is emitted with each
32-bit word reversed, taking HMAC-SHA1 with it.

libjuice uses HMAC-SHA1 for STUN MESSAGE-INTEGRITY, so on mips, mips64,
powerpc and every other big-endian target each ICE connectivity check is
rejected as failing integrity. No candidate pair is nominated and no
session can be established. picohash is the default backend, as USE_NETTLE
is off, so this affects the package as built here.

Verified on lantiq/xrx200 (mips_24kc, MIPS 34Kc): before the patch an ICE
agent loops with "STUN integrity check failed" and never leaves the
connecting state; after it the agent reaches completed and a session comes
up over the DHT and ICE. RFC 2202 HMAC-SHA1 test case 1 and the SHA-1
vector for "abc" both fail before and pass after.

Submitted upstream as paullouisageneau/libjuice#353 and, for the vendored
copy's origin, kazuho/picohash#13.

Signed-off-by: Daniel Golle <redacted>