packaging: cross-compile for the FP6's own cores

The phone is one SoC, 4x Cortex-A520 + 4x Cortex-A720, and the package is
built for nothing else, so build for it: -march=armv8.6-a+fp16fml+aes+sha3+sm4
-mtune=cortex-a720 instead of the generic Armv8 baseline. That is gcc's own
-march=native expansion on the phone, re-based on armv8.6-a so clang can
express the same set; 8.6 is the highest level whose mandatory set the phone
exposes (8.7 would assume WFxT, 9.x SVE2 - neither is in its hwcaps, and
SVE2 code would SIGILL). Same string as the fp6-img aports and kernel.

README.md

@@ -110,7 +110,7 @@ make install    # DESTDIR/PREFIX staged install incl. the packaging/ files
 packaging/make-sysroot.sh                  # one-time: Alpine aarch64 sysroot from the CDN
 crafter-build -- --target=aarch64-alpine-linux-musl \
     --sysroot=$HOME/.cache/imsd/sysroot-aarch64-alpine \
-    --march=armv8-a --mtune=generic
+    --march=armv8.6-a+fp16fml+aes+sha3+sm4 --mtune=cortex-a720
 # same flags after `crafter-build test --target=aarch64-alpine-linux-musl`
 # run the suite under qemu-aarch64 against the sysroot
 ```