The initial .text section generated in object files was missing the

SHF_ARM_PURECODE flag when being built with the -mexecute-only flag.
All code sections of an ELF must have the flag set for the final .text
section to be execute-only, otherwise the flag gets removed.

A HasData flag is added to MCSection to aid in the determination that
the section is empty. A virtual setTargetSectionFlags is added to
MCELFObjectTargetWriter to allow subclasses to set target specific
section flags to be added to sections which we then use in the ARM
backend to set SHF_ARM_PURECODE.

Patch by Ivan Lozano!

Reviewed By: echristo

Differential Revision: https://reviews.llvm.org/D48792

llvm-svn: 341593