[compiler-rt] Unify Linux and *BSD interceptors more

The Linux and *BSD interceptors are almost identical, except for *BSD,
where the overridden intercepted function is not defined weak due to
some incompliant linker behaviour.

Since most of the interception machinery is shared between Linux and
*BSD (see INTERCEPT_FUNCTION macro), it makes sense to unify interceptor
definition and declarations as much as possible to ease future changes.

NFC.

Reviewed By: dvyukov, vitalybuka

Differential Revision: https://reviews.llvm.org/D151318