Fix kernel -Werror support

The introduction of secondary arch support for kernel files disabled
-Werror for all kernel files, since the -Werror flags were moved from
{CC,C++}FLAGS to TARGET_WARNING_{CC,C++}FLAGS_<arch>, which, however,
was overwritten by the SetupKernel rule. This commit introduces new
global variables {HAIKU,HOST,TARGET}_WERROR_FLAGS[_<arch>], which
contain the additional -Werror flags to be applied for the architecture.
The config variable WARNINGS can be set to "treatAsErrors" to cause
-Werror and {HOST,TARGET}_WERROR_FLAGS[_<arch>] to be appended to the
compilation flags.

Fixes #10280.