Support arm32 R_ARM_V4BX relocation format

ARM32 ELF R_ARM_V4BX relocation format is a special relocation type
that records the location of an ARMv4t BX instruction to enable a
static linker to generate ARMv4 compatible instructions.  This
relocation does not contain a reference symbol.

This patch enabled its creation by removing the requeriment of a
relocation symbol target in ELFState<ELFT>::writeSectionContent.

llvm-svn: 235513