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>

arch/Config.in.arm

@@ -129,10 +129,6 @@ config BR2_arm926t
 	select BR2_ARM_CPU_MAYBE_HAS_VFPV2
 	select BR2_ARM_CPU_HAS_THUMB
 	select BR2_ARM_CPU_ARMV5
-config BR2_iwmmxt
-	bool "iwmmxt"
-	select BR2_ARM_CPU_HAS_ARM
-	select BR2_ARM_CPU_ARMV5
 config BR2_xscale
 	bool "xscale"
 	select BR2_ARM_CPU_HAS_ARM
@@ -817,7 +813,6 @@ config BR2_GCC_TARGET_CPU
 	default "strongarm"	if BR2_strongarm
 	# armv5
 	default "arm926ej-s"	if BR2_arm926t
-	default "iwmmxt"	if BR2_iwmmxt
 	default "xscale"	if BR2_xscale
 	# armv6
 	default "arm1136j-s"	if BR2_arm1136j_s