linux-user: detect mismatched ELF ABI in qemu-mips[n32][el]

linux-user: detect mismatched ELF ABI in qemu-mips[n32][el]

MIPS provides 2 ILP32 ABIs, and therefore 4 possible qemu-mips binaries
with 2 pairs using the same endianess and bitness.

This could lead to an O32 image loading in the N32 binary or vice versa
and in cryptic errors (if lucky that the CPU doesn't match the FPU used)
like :

  qemu: Unexpected FPU mode       (o32 ELF loaded to qemu-mipsn32[el])
  ELF binary's NaN mode not supported by CPU    (n32 -> qemu-mips[el])

Add an ABI check macro that could be used while checking the ELF header
that relies in the ABI2 flag to identify n32 binaries and abort instead
early with a more descriptive error :

  Invalid ELF image for this architecture

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200823101703.18451-1-carenas@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>