main/gcc: add symlink for ${CTARGET}cc

bootstrap has been broken in alpine for several weeks due to a change
in the recipe for `main/musl` to use `${CROSS_COMPILE}cc` instead of
`${CROSS_COMPILE}gcc`. this works for compiling on the local machine,
but breaks during bootstrap because there is no matching symlink from
${CTARGET}-cc to ${CTARGET}-gcc. this adds that symlink, fixing bootstrap.