LinuxKPI: Import vanilla linux/overflow.h

It is dual-licensed (GPLv2 & MIT) and self-contained header file.
No need to reimplement it.

Sponsored by: Serenity CyberSecurity, LLC
Reviewed by: emaste
MFC after: 1 week

sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

LinuxKPI: Constantly use _LINUXKPI_ prefix in include guards

MFC after: 1 week
Reviewed by: bz, emaste, hselasky, manu
Differential Revision: https://reviews.freebsd.org/D33562

<linux/overflow.h>: Don't use __has_builtin().

GCC only added support for __has_builtin in GCC 10. However, all
supported versions of GCC and clang include these builtins so just use
them unconditionally.

This fixes the build with GCC 9.

Reviewed by: manu, hselasky, imp
Differential Revision: https://reviews.freebsd.org/D31942

Implement the array_size() function in the LinuxKPI. This function
basically multiplies its two arguments and returns SIZE_MAX if the
result overflows the size_t type. Else the product of the two
arguments is returned.

Bump the FreeBSD_version to mitigate issues with existing
implementation of array_size() in drm-devel-kmod.

Discussed with: manu@
MFC after: 1 week
Sponsored by: Mellanox Technologies

linuxkpi: Add overflow.h

Only add check_add_overflow and check_mul_overflow as those are the only
two needed function by DRM v5.3.
Both gcc and clang have builtin to do this check so use them directly
but throw an error if the compiler/code checker doesn't support this builtin.

Sponsored-by: The FreeBSD Foundation
Reviewed by: hselsasky
Differential Revision: https://reviews.freebsd.org/D25015