linux-user: Move CPU type name selection to a function

linux-user: Move CPU type name selection to a function

Instead of a sequence of "#if ... #endif" move the
selection to a function in linux-user/*/target_elf.h

We can't add them in linux-user/*/target_cpu.h
because we will need to include "elf.h" to
use ELF flags with eflags, and including
"elf.h" in "target_cpu.h" introduces some
conflicts in elfload.c

Suggested-by: Richard Henderson <[email protected]>
Signed-off-by: Laurent Vivier <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20180220173307[email protected]>