configure, meson: move CPU_CFLAGS out of QEMU_CFLAGS

Flags that choose the target architecture, such as -m32 on x86, affect
all invocations of the compiler driver, for example including options
such as --print-search-dirs.  To ensure that they are treated as such,
place them in the cross file in the [binaries] section instead of
including them in QEMU_CFLAGS.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>