The `os_mremap` and `os_getpagesize` were missing in the `core/shared/platform/riot` and were preventing RIOT from compiling if using WAMR v2.x.x. This PR is adding both functions and should allow RIOT to update WAMR version to v2.x.x. Signed-off-by:: mewen.berthelot <mewen.berthelot@orange.com>