To reduce noise in the job log, silence the output of `git fetch`.

943315f4 (community/rust: add libssp_nonshared to link spec,
2020-02-08) fixed the linking issue, so now we can build on x86 again.

Fixes #11202

On musl libssp_nonshared needs to be explicitly added to provide
__stack_chk_fail_local (while rust does not use this symbol linking
in gcc built code can require it).

Latest version fails with:

```
note:
/usr/lib/gcc/i586-alpine-linux-musl/9.2.0/../../../../i586-alpine-linux-musl/bin/ld:
/tmp/rustclvHfVF/liblibgit2_sys-c1839c55229d18b6.rlib(refs.o): in function
`git_reference_normalize_name':
refs.c:(.text.git_reference_normalize_name+0xae): undefined reference to
`__stack_chk_fail_local'
```
(and more similar errors)

Instead of apk adding dependencies to sysroot, use the EXTRADEPENDS_TARGET
to pass in these dependencies. This does not leave the sysroot in
dirty state, but instead cleans it up.

Current abuild has a bug that when CC is defined, it's used to
guess build architecture. But during cross-compile setup it points
to cross compiler and causing breakage. Export CBUILD after initial
abuild functions.sh run to workaround this.

That seems to require D cross-compiler to work, which was just
disabled.

Oleg Titov authored 5 years ago and Leo committed 5 years ago

Ref https://github.com/xiph/rav1e/releases