[safestack] Remove pageSize

Summary:
3rd party sysconf interceptor may crash if it's called before unsafe_stack_setup

However pageSize is not useful here. mmap should round up on it's own, SFS_CHECK can be removed.

Reviewers: eugenis, vlad.tsyrklevich

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D57924

llvm-svn: 353481