scripts/Makefile.extrawarn: Respect CONFIG_WERROR / W=e for hostprogs

scripts/Makefile.extrawarn: Respect CONFIG_WERROR / W=e for hostprogs

Commit 27758d8c2583 ("kbuild: enable -Werror for hostprogs")
unconditionally enabled -Werror for the compiler, assembler, and linker
when building the host programs, as the build footprint of the host
programs is small (thus risk of build failures from warnings are low)
and that stage of the build may not have Kconfig values (thus
CONFIG_WERROR could not be used as a precondition).

While turning warnings into errors unconditionally happens in a few
places within the kernel, it can be disruptive to people who may be
building with newer compilers, such as while doing a bisect. While it is
possible to avoid this behavior by passing HOSTCFLAGS=-w or
HOSTCFLAGS=-Wno-error, it may not be the most intuitive for regular
users not intimately familiar with Kbuild.

Avoid being disruptive to the entire build by depending on the explicit
opt-in of CONFIG_WERROR or W=e to enable -Werror and the like while
building the host programs. While this means there is a small portion of
the build that does not have -Werror enabled (namely scripts/kconfig/*
and scripts/basic/fixdep), it is better than not having it altogether.

Fixes: 27758d8c2583 ("kbuild: enable -Werror for hostprogs")
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Reported-by: Askar Safin <safinaskar@gmail.com>
Closes: https://lore.kernel.org/20251005011100.1035272-1-safinaskar@gmail.com/
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Tested-by: Miguel Ojeda <ojeda@kernel.org> # Rust
Link: https://patch.msgid.link/20251006-kbuild-hostprogs-werror-fix-v1-1-23cf1ffced5c@kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>