profiles: add default llvm_targets for rust
It's not installable without user invervention
with latest changes.
!!! The ebuild selected to satisfy "dev-lang/rust" has unmet
requirements.
- dev-lang/rust-1.26.0::gentoo USE="-debug -doc (-extended) -jemalloc"
ABI_X86="(64)" LLVM_TARGETS="-AArch64 -AMDGPU -ARM -BPF -Hexagon -Lanai
-MSP430 -Mips -NVPTX -PowerPC -Sparc -SystemZ -X86 -XCore"
Treat it the same way as llvm/clang does.
Not touching musl/uclibc profiles, since they
maintain their own rust ebuild.
Fixes: https://bugs.gentoo.org/655780