lang/rust: fix build on powerpc64 elfv2

After upgrade to LLVM 10, core in stage 1 fails to compile (no clear reason, rustc crashes):
pid 26828 (rustc), jid 0, uid 0: exited on signal 11 (core dumped)

Compilation with GCC works fine, so switch to GCC for the time being.

PR: 244813
Approved by: mikael