Investigate failures to link in standard libraries

[adpaco]

[nchong-at-aws]

cargo new SizeAndAlignOfDstTest #< this test needs mutex and cell (which aren't linked)
cd SizeAndAlignOfDstTest
cp ~/rmc/rust-tests/cbmc-reg/SizeAndAlignOfDst/main_fail.rs src/main.rs 
rustup component add rust-src --toolchain nightly
cargo +nightly run -Z build-std --target x86_64-unknown-linux-gnu #< succeeds (just compiling)
RUSTFLAGS="-Z trim-diagnostic-paths=no -Z codegen-backend=gotoc --cfg=rmc" RUSTC=rmc-rustc cargo +nightly build -Z build-std --target x86_64-unknown-linux-gnu #< fails trying to compile libc