xen/build: Use C99 booleans

xen/build: Use C99 booleans

and switch bool_t to being of type _Bool rather than char.

Using bool_t as char causes several subtle problems; first that a bool_t
actually has more than two values, and that (bool_t)0x100 actually has the
value 0 rather than the expected 1, due to truncation.

Making this change reveals two bugs now caught by the compiler.
errata_c6_eoi_workaround() actually makes use of bool_t having more than two
states, while generic_apic_probe() has a integer in the middle of a compound
bool_t assignment (which triggers a [-Werror=parentheses] warning on Debian
Jessie).

Finally, it turns out that ARM is mixing and matching bool_t and bool, despite
their different semantics.  This change brings the semantics of bool_t to
match bool, but does not alter the current mix.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Acked-by: Tim Deegan <tim@xen.org>