configure: Fix endianess test with LTO

configure: Fix endianess test with LTO

If a user is trying to compile QEMU with link-time optimization
enabled by running the configure script like this:

 .../configure --extra-cflags="-flto"

then the endianess test is failing since the magic values do not
show up in the intermediate object files there. If the host is
a big endian machine (like s390x), the QEMU binary is then unusable
since the corresponding variable "bigendian" is pre-initialized
with "no".

To fix this issue, we should rather create a full binary and look
for the magic strings there instead.
And we really should not continue the build if the endianess check
failed, to make it clear right from the start that something went
wrong here, thus let's also add some "exit 1" statements here
after emitting the error message.

Message-Id: <20210715083928.933806-1-thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>