tcg/i386: Use GDB JIT debugging interface only for hosts with ELF

Not all i386 / x86_64 hosts use ELF.
Ask the compiler whether ELF is used.

On w64, gdb crashes when ELF_HOST_MACHINE is defined.

Cc: Blue Swirl <blauwirbel@gmail.com>
Acked-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
