Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Provide sysarch(2) prototypes in the MD sysarch.h headers. While I'm
at it, use the ANSI C generic pointer type for the second argument,
thus matching the documentation.

Remove the now extraneous (and now conflicting) function declarations
in various libc sources. Remove now unnecessary casts.

Reviewed by: bde

Add implementations of amd64_[gs]et_[fg]sbase().