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

[brooks]

Summary

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.

This reverts commit 029a09f18032353a9ae874590b879322efc6e53a
This reverts commit 19728f31ae421f40e2b0b0c775f4eedd7f927be0

Effort: CHERI upstreaming
Fixes: 85ab981a8e4e ("sys/_types.h: define fallback __(u)intcap_t")
Sponsored by: Innovate UK

[brooks]

sys/sys/_types.h

164

The most optimal gate would probably be !defined(__INTCAP_TYPE__) since it's a fundamental type provided by the compiler, but we seem to have forgotten to define that macro. I've filed a bug report on that lack https://github.com/CTSRD-CHERI/llvm-project/issues/796

[brooks]

Switch to checking for SIZEOF_INTCAP. The compiler will define it
if it implements __intcap_t.