mips: increase stack alignment in clone to match the ABI

In "mips: align stack in clone [BZ #28223]"
(commit 1f51cd9a) I made a mistake: I
misbelieved one "word" was 2-byte and "doubleword" should be 4-byte.
But in MIPS ABI one "word" is defined 32-bit (4-byte), so "doubleword" is
8-byte [1], and "quadword" is 16-byte [2].

[1]: "System V Application Binary Interface: MIPS(R) RISC Processor
      Supplement, 3rd edition", page 3-31
[2]: "MIPSpro(TM) 64-Bit Porting and Transition Guide", page 23