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

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

Change the return type to abi_ulong, and pass in the cpu.
Duplicate the one line function between i386 and x86_64,
as most other additions to elfload.c won't be common.

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