[VE] Support relocation information in MC layer

Summary:
Change VEAsmParser to support identification with relocation information
in assmebler.  Change VEAsmBackend to support relocation information in
MC layer.  Change VEDisassembler and VEMCCodeEmitter to support binary
generation of branch target operands.  Add REFLONG fixup and variant kind
to support new R_VE_REFLONG ELF symbol.  And, add regression test in both
MC and CodeGen to check binary genaration with relocation information.

Differential Revision: https://reviews.llvm.org/D81553