sys/_types.h: avoid use of __has_feature()

Only fairly recent GCC versions support and sys/_types.h must work with
quite old compilers and without sys/cdef.h being included. The prior
workaround works fine, but we can have the same effect with compiler
macro definitions. In this specific case, compilers that define the
intcap_t builtin type will define SIZEOF_INTCAP__.

This reverts commit 029a09f18032353a9ae874590b879322efc6e53a
This reverts commit 19728f31ae421f40e2b0b0c775f4eedd7f927be0

Reviewed by: imp, des, kib, emaste
Effort: CHERI upstreaming
Fixes: 85ab981a8e4e ("sys/_types.h: define fallback __(u)intcap_t")
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D54009