[SKIP SOF-TEST] llext: add a comment where builtin functions are exported #19974

Job	Run time
build-windows (imx8 imx8x imx8m imx8ulp)	9m 4s
build-linux (mnfst, imx8 imx8x imx8m imx8ulp)	6m 50s
build-windows (mtl lnl)	7m 39s
LP64-WIP	4m 7s
build-linux (mnfst, mtl lnl)	5m 53s
build-windows (ptl)	5m 40s
build-linux (mnfst, ptl)	5m 5s
build-windows (tgl tgl-h)	7m 16s
build-linux (mnfst, tgl tgl-h)	5m 49s
manifest-check	52s
build-windows (-d, lnl)	5m 58s
build-linux (zmain, imx8 imx8x imx8m imx8ulp)	6m 43s
build-linux (zmain, mtl lnl)	5m 48s
build-linux (zmain, ptl)	4m 31s
build-linux (zmain, tgl tgl-h)	5m 48s
build-linux (-d, mnfst, lnl)	4m 34s
build-linux (mnfst, --all)	14m 33s
compare-linux-win	1m 10s
	1h 47m 20s

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