rust: rustdoc reproducibility issue fix - disable PGO

rust: rustdoc reproducibility issue fix - disable PGO

The PGO (Profile-guided Optimization) collect data about the typical execution of a program
and then use this data to inform optimizations such as inlining, machine-code layout,
register allocation, etc.

This optimization is by default disabled in rust sources but enabled in Yocto and causing
the reproducibility issue in rustdoc binary. To fix the issue this optimization is set to
it's default 'false'.

More about the optimization: https://doc.rust-lang.org/rustc/profile-guided-optimization.html

With the reproducibility issue fixed, we can enable the reproducibility tests again.

Signed-off-by: Sundeep KOKKONDA <sundeep.kokkonda@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>