aportgen: gcc cross packages shouldn't build libgcc

In order to fix #2167 (closed), I did the following:

• remove isl from depends (!2203 (merged))
• regenerate the cross/gcc-* packages
• build gcc-aarch64 successfully
• try pmbootstrap build --arch=aarch64 --force hello-world

it fails with:

(077758) [15:22:51] (buildroot_aarch64) % apk --no-progress add -u --virtual .pmbootstrap /mnt/pmbootstrap-packages/aarch64/libgcc-12.1.1_git20220630-r5.apk
ERROR: unable to select packages:
  libgcc-12.1.1_git20220630-r5:
    conflicts: libgcc-12.1.1_git20220630-r5
    satisfies: .pmbootstrap-20220911.132252[libgcc><Q1+tYoegZ+gQVuxXlvuBda6e6EOps=]
  .pmbootstrap-20220911.132252:
    masked in: cache
    satisfies: world[.pmbootstrap=20220911.132252]
  libgcc-12.1.1_git20220630-r5:
    conflicts: libgcc-12.1.1_git20220630-r5
    breaks: .pmbootstrap-20220911.132252[libgcc><Q1+tYoegZ+gQVuxXlvuBda6e6EOps=]
    satisfies: gcc-12.1.1_git20220630-r5[so:libgcc_s.so.1]
               binutils-2.39-r1[so:libgcc_s.so.1]
               libstdc++-12.1.1_git20220630-r5[so:libgcc_s.so.1]
               ccache-4.6.3-r0[so:libgcc_s.so.1]
               lzip-1.23-r0[so:libgcc_s.so.1]

what happens is, that the current gcc-aarch64 and similar packages build libgcc too, and they get stored in the local package repository for aarch64 packages. whereas the gcc-aarch64 and g++-aarch64 packages go into the local x86_64 repository.

apparently this does not happen once the package is built via bpo and published, since only the x86_64 packages get uploaded back to bpo.

workaround:

• remove libgcc package in ~/.local/var/pmbootstrap/packages/edge/aarch64
• "pmbootstrap index" to update the local APKINDEX

to fix this, pmbootstrap should probably set BOOTSTRAP=nolibc while generating the apkbuild, which will result in no libgcc getting built. Should be easy to fix, but creating this issue for reference.