Fixes: thread 'main' panicked at 'failed to gather the target spec
for '<arch>-unknown-linux-gnu', synthetic_targets.rs:66:9
Detect and fetch custom target configurations when rustc is
bootstrapped in rust oe-selftest.
(From OE-Core rev: fdd9385d9845d628e10179598fc42d29519c5dfd)
Signed-off-by: Yash Shinde <Yash.Shinde@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
