kbuild: enable -Werror for hostprogs

The hostprog compilers and linkers do not share the regular compiler flags,
so they are not affected by CONFIG_WERROR or W=e. As hostprogs are used
during the bootstrap of the build, they can't depend on kconfig options.

Enable -Werror unconditionally.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://lore.kernel.org/rust-for-linux/CANiq72k-PdSH2BNgbq=X+FhpyEErifSCKfO5ObXz6bu9_J8+fA@mail.gmail.com/
Link: https://lore.kernel.org/r/20250814-kbuild-werror-v2-6-c01e596309d2@linutronix.de
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: t-8ch

scripts/Makefile.extrawarn

@@ -224,3 +224,8 @@ KBUILD_USERLDFLAGS	+= -Wl,--fatal-warnings
 KBUILD_RUSTFLAGS	+= -Dwarnings
 
 endif
+
+# Hostprog flags are used during build bootstrapping and can not rely on CONFIG_ symbols.
+KBUILD_HOSTCFLAGS	+= -Werror
+KBUILD_HOSTLDFLAGS	+= -Wl,--fatal-warnings
+KBUILD_HOSTRUSTFLAGS	+= -Dwarnings