configure: fix parameter expansion of --cross-cc-cflags options

configure: fix parameter expansion of --cross-cc-cflags options

Without this fix, any use of --cross-cc-cflags-* causes a message like:
$ ../configure --cross-cc-ppc64le=clang --cross-cc-cflags-ppc64le="-target powerpc64le-unknown-linux-gnu -sysroot ..."
../configure: 1: eval: cross_cc_cflags_--cross-cc-cflags-ppc64le=-target: not found
../configure: 3816: export: cross_cc_cflags_--cross-cc-cflags-ppc64le: bad variable name

Signed-off-by: Matheus Ferst <matheus.ferst@eldorado.org.br>
Message-Id: <20220120173142.2755077-1-matheus.ferst@eldorado.org.br>
[Fix other occurrences too, noted by Philippe Mathieu-Daudé. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>