openssh: fix sshd for MIPS64 n32

openssh: fix sshd for MIPS64 n32

This patch backports two patches that have been sent upstream as a pull
request in order to fix sshd for MIPS64 n32.

The first patch adds support for detecting the MIPS ABI during the
configure phase.

The second patch sets the right value to seccomp_audit_arch taking into
account the MIPS64 ABI.

Currently seccomp_audit_arch is set to AUDIT_ARCH_MIPS64 or
AUDIT_ARCH_MIPSEL64 (depending on the endinness) when openssh is built
for MIPS64. However, that's only valid for n64 ABI. The right macros for
n32 ABI defined in seccomp.h are AUDIT_ARCH_MIPS64N32 and
AUDIT_ARCH_MIPSEL64N32, for big and little endian respectively.

Because of that an sshd built for MIPS64 n32 rejects connection attempts
and the output of strace reveals that the problem is related to seccomp
audit:

[pid   194] prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, {len=57,
filter=0x555d5da0}) = 0
[pid   194] write(7, "\0\0\0]\0\0\0\5\0\0\0Ulist_hostkey_types: "..., 97) = ?
[pid   193] <... poll resumed> )        = 2 ([{fd=5, revents=POLLIN|POLLHUP},
{fd=6, revents=POLLHUP}])
[pid   194] +++ killed by SIGSYS +++

Pull request: https://github.com/openssh/openssh-portable/pull/71

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>