Revert "Replace CMAKE_SYSROOT_FLAG with a lang specific COMPILE_OPTION."

This reverts commit 1dd9dd2c0b626ac011d792f7711bc7c5c2dbfc01.
Merge topic 'replace-CMAKE_SYSROOT_FLAG' into next

86707de Revert "Replace CMAKE_SYSROOT_FLAG with a lang specific COMPILE_OPTION."