[ARM][AArch64] Workaround ARM/AArch64 peculiarity in clearing icache.

Certain ARM implementations treat icache clear instruction as a memory read,
and CPU segfaults on trying to clear cache on !PROT_READ page.
We workaround this in Memory::protectMappedMemory by adding
PROT_READ to affected pages, clearing the cache, and then setting
desired protection.

This fixes "AllocationTests/MappedMemoryTest.***/3" unit-tests on
affected hardware.

Reviewers: psmith, zatrazz, kristof.beyls, lhames

Reviewed By: lhames

Subscribers: llvm-commits, krytarowski, peter.smith, jgreenhalgh, aemerson,
             rengolin

Patch by maxim-kuvrykov! 

Differential Revision: https://reviews.llvm.org/D40423

llvm-svn: 319166