heap: simplify heap size calculation #22880

Job	Run time
manifest-check	2m 42s
LP64-WIP	3m 50s
build-windows (mnfst, lnl)	8m 40s
build-windows (mnfst, imx8m imx8ulp)	9m 7s
build-windows (mnfst, imx8 imx8x)	9m 31s
build-windows (mnfst, tgl-h)	8m 23s
build-linux (mnfst, lnl)	4m 26s
build-linux (mnfst, ptl)	5m 43s
build-linux (mnfst, imx8m imx8ulp)	4m 46s
build-windows (mnfst, tgl)	8m 1s
build-linux (mnfst, tgl)	4m 54s
build-windows (mnfst, ptl)	11m 9s
build-linux (mnfst, tgl-h)	4m 21s
build-linux (mnfst, imx8 imx8x)	5m 5s
build-linux (mnfst, wcl imx95)	6m 58s
build-linux (mnfst, mtl)	4m 36s
build-windows (-d, mnfst, lnl)	10m 10s
build-windows (mnfst, mtl)	11m 21s
build-linux (-d, mnfst, lnl)	4m 30s
compare-linux-win	33s
	2h 8m 46s

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