buildroot: mark ia64, m68k, nios2, sparc and sparc64 as broken

No sense in showing the architecture options when they cannot even build
the toolchain.

target/Config.in.arch

@@ -13,21 +13,23 @@ config BR2_armeb
 config BR2_avr32
 	bool "avr32"
 	select BR2_SOFT_FLOAT
-
 config BR2_cris
 	bool "cris"
 config BR2_ia64
 	bool "ia64"
+	depends on BROKEN # uclibc linker errors
 config BR2_i386
 	bool "i386"
 config BR2_m68k
 	bool "m68k"
+	depends on BROKEN # ice in uclibc / inet_ntoa_r
 config BR2_mips
 	bool "mips"
 config BR2_mipsel
 	bool "mipsel"
 config BR2_nios2
 	bool "nios2"
+	depends on BROKEN # no kernel headers
 config BR2_powerpc
 	bool "powerpc"
 config BR2_sh
@@ -36,8 +38,10 @@ config BR2_sh64
 	bool "superh64"
 config BR2_sparc
 	bool "sparc"
+	depends on BROKEN # uclibc linker errors
 config BR2_sparc64
 	bool "sparc64"
+	depends on BROKEN # uclibc libm linker errors
 config BR2_x86_64
 	bool "x86_64"
 endchoice