arch/Config.in.arm: drop support for iwmmxt architecture variant

This architecture variant is broken in GCC, causing build failures:

../../../libgcc/config/arm/unwind-arm.c:467:1: error: unrecognizable insn:
  467 | }
      | ^
(insn 2 4 3 2 (set (reg/v/f:SI 118 [ p ])
        (reg:SI 0 r0 [ p ])) "../../../libgcc/config/arm/unwind-arm.c":456:1 -1
     (nil))
during RTL pass: vregs
../../../libgcc/config/arm/unwind-arm.c:467:1: internal compiler error: in extract_insn, at recog.c:2770

Reported to the GCC developers, the feedback was "iwmmxt support is
definitely bitrotten and most likely should be removed from GCC.".

See:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106972#c1

So this commit simply drops support for iwmmxt, which anyway is
probably barely used nowadays: it's for old Marvell PXA cores that
implemented a special SIMD instruction set. The BR2_xscale option can
be used instead, it's just that it won't use this SIMD instruction
set.

Fixes:

  http://autobuild.buildroot.net/results/8e4c4512902c34d8ec0c6f8dfff92b7a198e4b4a/

and the numerous other build failures at:

  http://autobuild.buildroot.net/?reason=host-gcc-initial%&subarch=iwmmxt

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

Config.in.legacy

@@ -146,6 +146,16 @@ endif
 
 comment "Legacy options removed in 2022.11"
 
+config BR2_iwmmxt
+	bool "ARM iwmmxt variant removed"
+	select BR2_LEGACY
+	help
+	  Support for the ARM iwmmxt architecture variant in GCC has
+	  bitroten and is no longer maintained. GCC maintainers
+	  recommend to no longer use it, and suggest to use "xscale"
+	  as a replacement architecture variant. See
+	  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106972
+
 config BR2_PACKAGE_GPSD_OLDSTYLE
 	bool "gpsd oldstyle removed"
 	select BR2_LEGACY