Resolve GCC Cross-compilation Issues

[stux2000]

I'm creating this issue to keep track of problems when cross-compiling Minix, particularly on Linux platforms

• pull request llvm: add static_cast to MDMap return #310
• https://groups.google.com/g/minix3/c/A_VKJdj_Pa8
• Issues compile error on Linux/GCC, build.sh #309, Cross-compiling MINIX 3 on Debian Stretch doesn't work #229 (resolved by pull request Correct Debian 10 cross compile issue. #301)
• https://groups.google.com/g/minix3/c/wF5ZdsbgLSc -- building with GCC 10/Clang 11

NOTE: Please do not comment on new issues here, instead please submit to a new issue and link to this one.

[petershh]

I'll try to describe current state of things here.

Currently, build is successful with GCC 8.

Firstly, there are links with breaking changes in GCC 9 and GCC 10:

• https://gcc.gnu.org/gcc-9/porting_to.html
• https://gcc.gnu.org/gcc-10/porting_to.html

It seems that Clang mimics GCC behavior, so information about these changes remains correct for Clang.

How these changes affect us:

1. Nbmake fails to build because -fno-common flag is default now. This can be fixed rather easily. Also, we can take nbmake from NetBSD sources
2. Cross tools from /external like binutils cannot be built. Currently, build process uses obsolete versions of these tools, so they should be upgaded. But this is a hard task because there is a lot of patches to make these tools work with NetBSD and MINIX. I don't think it is possible to upgrade these patches without experienced MINIX developer.