PPC64LE: Improve `poly_tomont`

[mkannwischer]

Optimization deferred from #1648.

See

mlkem-native/dev/ppc64le/src/poly_tomont_ppc_asm.S

Lines 110 to 120 in 494dfe6

	* TODO: this saves/restores v20-v30, but the body only touches
	* v23 (vdata3), v24 (vtmp3) and v27-v30 (vresult1..4) in the
	* callee-saved range. v20, v21, v22, v25, v26 are spilled and
	* reloaded but never used -- 5 dead stxvx/lxvx pairs plus 80
	* bytes of frame. Trim the save set (and frame size) to the
	* registers actually clobbered. Separate change from the doc
	* pass, since it alters emitted code.
	*/
	li 6, 128
	li 7, 144
	li 8, 160