CI: build with a single codegen unit

This is not ideal, but it does provide two things:

1. It fixes bizarre linker errors about missing `sancov` symbols.

2. It allows LLVM to do inlining that it otherwise refuses to do. For some
reason, when sanitizers are enabled, LLVM refuses to inline across codegen
units. This is a problem because trivial methods like `Vec::len` won't be
inlined, resulting in 100x slowdowns.

`cargo fuzz` already restricts its builds to a single codegen unit, so we might
as well do the same thing in CI here.

ci/script.sh

@@ -11,6 +11,7 @@ pushd ./example
 cargo rustc \
       --release \
       -- \
+      -Ccodegen-units=1 \
       -Cpasses=sancov-module \
       -Cllvm-args=-sanitizer-coverage-level=3 \
       -Cllvm-args=-sanitizer-coverage-trace-compares \
@@ -26,6 +27,7 @@ pushd ./example_arbitrary
 cargo rustc \
       --release \
       -- \
+      -Ccodegen-units=1 \
       -Cpasses=sancov-module \
       -Cllvm-args=-sanitizer-coverage-level=3 \
       -Cllvm-args=-sanitizer-coverage-trace-compares \
@@ -46,6 +48,7 @@ pushd ./example_mutator
 cargo rustc \
       --release \
       -- \
+      -Ccodegen-units=1 \
       -Cpasses=sancov-module \
       -Cllvm-args=-sanitizer-coverage-level=3 \
       -Cllvm-args=-sanitizer-coverage-trace-compares \