linux-user: ARM-FDPIC: Identify ARM FDPIC binaries

Define an ARM-specific version of elf_is_fdpic:
FDPIC ELF objects are identified with e_ident[EI_OSABI] ==
ELFOSABI_ARM_FDPIC.

Co-Authored-By: Mickaël Guêné <mickael.guene@st.com>
Signed-off-by: Christophe Lyon <christophe.lyon@st.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20180430080404.7323-3-christophe.lyon@st.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>