heap: simplify heap size calculation #22837

Job	Run time
LP64-WIP	4m 21s
build-windows (-d, lnl)	9m 20s
build-windows (mtl)	10m 30s
build-windows (ptl)	10m 40s
manifest-check	2m 54s
build-windows (tgl-h)	9m 34s
build-linux (mnfst, lnl)	4m 35s
build-linux (mnfst, imx8 imx8x)	5m 29s
build-linux (-d, mnfst, lnl)	4m 31s
build-linux (mnfst, tgl-h)	5m 14s
build-windows (imx8m imx8ulp)	10m 16s
build-windows (lnl)	9m 42s
build-windows (tgl)	8m 55s
build-linux (mnfst, mtl)	4m 40s
build-linux (mnfst, tgl)	4m 52s
build-linux (mnfst, wcl imx95)	7m 20s
build-windows (imx8 imx8x)	10m 4s
build-linux (mnfst, imx8m imx8ulp)	5m 18s
build-linux (mnfst, ptl)	5m 35s
compare-linux-win	34s
	2h 14m 24s

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