kbuild: pacman-pkg: make "rc" releases adhere to pacman versioning scheme

The package versioning scheme does not enable smooth upgrades from "rc"
releases to the corresponding stable releases (e.g. 7.0.0-rc7 -> 7.0.0)
because pacman considers that a downgrade due to the underscore in
pkgver (e.g. 7.0.0_rc7), see e.g. vercmp(8) for an explanation of the
package version comparison used by pacman. Package versions which are
derived from said releases (e.g. built from git revisions) are
similarly affected. Fix this by modifying pkgver in order to remove the
hyphen from kernel versions containing "-rcN", where N is a
non-negative integer.

Acked-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Viktor Jägersküpper <viktor_jaegerskuepper@freenet.de>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260515215913.92481-1-viktor_jaegerskuepper@freenet.de


Fixes: c8578539 ("kbuild: add script and target to generate pacman package")
Signed-off-by: Nicolas Schier <nsc@kernel.org>