rust: suppress error messages from CONFIG_{RUSTC,BINDGEN}_VERSION_TEXT

While this is a somewhat unusual case, I encountered odd error messages
when I ran Kconfig in a foreign architecture chroot.

  $ make allmodconfig
  sh: 1: rustc: not found
  sh: 1: bindgen: not found
  #
  # configuration written to .config
  #

The successful execution of 'command -v rustc' does not necessarily mean
that 'rustc --version' will succeed.

  $ sh -c 'command -v rustc'
  /home/masahiro/.cargo/bin/rustc
  $ sh -c 'rustc --version'
  sh: 1: rustc: not found

Here, 'rustc' is built for x86, and I ran it in an arm64 system.

The current code:

  command -v $(RUSTC) >/dev/null 2>&1 && $(RUSTC) --version || echo n

can be turned into:

  command -v $(RUSTC) >/dev/null 2>&1 && $(RUSTC) --version 2>/dev/null || echo n

However, I did not understand the necessity of 'command -v $(RUSTC)'.

I simplified it to:

  $(RUSTC) --version 2>/dev/null || echo n

Fixes: 2f7ab126

 ("Kbuild: add Rust support")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20240727140302.1806011-1-masahiroy@kernel.org


[ Rebased on top of v6.11-rc1. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>