translate-all: Avoid -Werror=switch-bool

gcc 5.3.0 diagnoses

translate-all.c: In function ‘alloc_code_gen_buffer’:
translate-all.c:756:17: error: switch condition has boolean value
         switch (buf2 != MAP_FAILED) {
                 ^

Signed-off-by: Richard Henderson <rth@twiddle.net>