[JITLink] Introduce target flags for Symbol and prepare ObjectLinkingLayer to account...

[JITLink] Introduce target flags for Symbol and prepare ObjectLinkingLayer to account for them

AArch32 branch offsets explicitly encode the target instruction subset (Arm/Thumb) in their least significant bit. We want this bit set (or clear) in addreses we hand out, but the addresses in the LinkGraph should be the real/physical addresses.

This patch allows ELFLinkGraphBuilder's to set target-specific flags in jitlink::Symbol and prepares ObjectLinkingLayer to account for them.

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D146641