Fix installation of clang in gentoo install script (#24917)

fix: sys-devel/clang was renamed to llvm-core/clang

see: https://github.com/gentoo/gentoo/commit/1f9f1999cdc8ccb94054dec2d2951c7e486aa996

util/install/gentoo.sh

@@ -22,7 +22,7 @@ _qmk_install() {
     echo "sys-devel/gcc multilib\ncross-arm-none-eabi/newlib nano" | sudo tee --append /etc/portage/package.use/qmkfirmware >/dev/null
     sudo emerge -auN sys-devel/gcc
     sudo emerge -au --noreplace \
-        app-arch/unzip app-arch/zip net-misc/wget sys-devel/clang \
+        app-arch/unzip app-arch/zip net-misc/wget llvm-core/clang \
         sys-devel/crossdev \>=dev-lang/python-3.7 dev-embedded/avrdude \
         dev-embedded/dfu-programmer app-mobilephone/dfu-util sys-apps/hwloc \
         dev-libs/hidapi