[scudo/standalone] Use zx_system_get_page_size() on Fuchsia

Fuchsia is migrating to a variable page size.

Reviewed By: hctim

Differential Revision: https://reviews.llvm.org/D95921

GitOrigin-RevId: d81069e796f76abc3747af646c9b6176e4a91de7
Change-Id: Ic3aa6a74ec07b26b89f6327b42db753372ff962f
1 file changed