linux-user: Move get_elf_platform to {i386,x86_64}/elfload.c

Move get_elf_platform to i386/elfload.c; pass in CPUState.
Create a simple get_elf_platform for x86_64.
Introduce HAVE_ELF_PLATFORM.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
6 files changed