services/qemu-binfmt: Use the F flag and the static output of QEMU.

Fixes <https://issues.guix.gnu.org/36117>.

Before this change, the 'binfmt_misc' entries registered for QEMU would not be
usable in container contexts outside of guix-daemon (without manually bind
mounting file names).

For example:

$ docker run --rm arm32v7/debian true
standard_init_linux.go:207: exec user process caused "no such file or directory"

After this change, any container can make use of the QEMU binfmt_misc
registrations, as their corresponding QEMU static binaries are fully
pre-loaded by the kernel.

* gnu/services/virtualization.scm (<qemu-platform>): Define using
'define-record-type*'.
[flags]: New field, which defaults to "F" (fix binary).
(%i386, %i486, %alpha, %arm, %armeb, %sparc, %sparc32plus, %ppc, %ppc64)
(%ppc64le, %m68k, %mips, %mipsel, %mipsn32, %mipsn32el, %mips64, %mips64el)
(%riscv32, %riscv64, %sh4, %sh4eb, %s390x, %aarch64, %hppa): Adjust.
(qemu-binfmt-guix-chroot): Remove variable.
(qemu-binfmt-service-type): Remove the qemu-binfmt-guix-chroot extension.
* gnu/services/qemu-binfmt (qemu-platform->binfmt):  Use the static output of
QEMU.
* doc/contributing.texi (Submitting Patches): Update doc.
* doc/guix.texi (Virtualization Services): Update doc.

doc/contributing.texi

@@ -1016,7 +1016,6 @@ your @code{operating-system} configuration:
 (service qemu-binfmt-service-type
  (qemu-binfmt-configuration
    (platforms (lookup-qemu-platforms "arm" "aarch64"))
-   (guix-support? #t)))
 @end lisp
 
 Then reconfigure your system.

doc/guix.texi

@@ -28723,13 +28723,6 @@ This is the configuration for the @code{qemu-binfmt} service.
 The list of emulated QEMU platforms.  Each item must be a @dfn{platform
 object} as returned by @code{lookup-qemu-platforms} (see below).
 
-@item @code{guix-support?} (default: @code{#t})
-When it is true, QEMU and all its dependencies are added to the build
-environment of @command{guix-daemon} (@pxref{Invoking guix-daemon,
-@option{--chroot-directory} option}).  This allows the @code{binfmt_misc}
-handlers to be used within the build environment, which in turn means
-that you can transparently build programs for another architecture.
-
 For example, let's suppose you're on an x86_64 machine and you have this
 service:
 
@@ -28737,7 +28730,6 @@ service:
 (service qemu-binfmt-service-type
          (qemu-binfmt-configuration
            (platforms (lookup-qemu-platforms "arm"))
-           (guix-support? #t)))
 @end lisp
 
 You can run:
@@ -28752,10 +28744,6 @@ build}, transparently using QEMU to emulate the ARMv7 CPU@.  Pretty handy
 if you'd like to test a package build for an architecture you don't have
 access to!
 
-When @code{guix-support?} is set to @code{#f}, programs for other
-architectures can still be executed transparently, but invoking commands
-like @command{guix build -s armhf-linux @dots{}} will fail.
-
 @item @code{qemu} (default: @code{qemu})
 The QEMU package to use.
 @end table