mips: pass 7th argument in syscalls

mips: pass 7th argument in syscalls

Only arg1 to arg6 have been passed down to kernel for syscalls.
This patch ensures that arg7 is also passed down for syscalls.
In addition to this, ensure that we have 16-byte aligned stack during
mips64 syscall.

Along with the change for sync_file_range, this will fix sync_file_range01
failure within LTP test suite.

Patch by Nikola Milutinovic.