devel/wasi-libc: fix build when ${FLAVOR} is not specified

To have separate flavours per supported LLVM version, USES=llvm is
version-locked to each flavour as USES=llvm:${FLAVOR}. This allows
${LLVM_VERSION} to be used throughout the Makefile for consistency.
However, when ${FLAVOR} is not specified, ${LLVM_VERSION} uses
${LLVM_DEFAULT} which may not match the first item in ${FLAVORS},
resulting in missing dependency. Specify a fallback ${FLAVOR} when
not otherwise set so USES=llvm is guaranteed a version lock.

Consumers always explicitly specify ${FLAVOR} so this problem is
not generally exposed.

Reported by: dim
PR: 282265

(cherry picked from commit c0c57c629e25107ee5ead1f631fce3586280845f)