Disable context functions for Thumb

They're written in ARM dialect, and `ldmia r14, {r14, pc}` is not accepted in T2
encoding. GCC8 changed the list of multilibs for arm-*, which now includes a
variant with CPU that supports T2 but not A1 encoding.

Signed-off-by: Alexey Neyman <stilor@att.net>

scripts/build/arch/arm.sh

@@ -96,6 +96,7 @@ CT_DoArchUClibcCflags() {
         case "${f}" in
             -mthumb)
                 CT_KconfigEnableOption "COMPILE_IN_THUMB_MODE" "${cfg}"
+                CT_KconfigDisableOption "UCLIBC_HAS_CONTEXT_FUNCS" "${cfg}"
                 ;;
             -marm)
                 CT_KconfigDisableOption "COMPILE_IN_THUMB_MODE" "${cfg}"