arm/thumb: outputs an implicit IT block to avoid build break

Fix build break on thumb2 mode:
opus/celt/arm/celt_pitch_xcorr_arm-gnu.S: Assembler messages:
opus/celt/arm/celt_pitch_xcorr_arm-gnu.S:146: Error: thumb conditional instruction should be in IT block -- `movle pc,lr'

Reference:
https://developer.arm.com/documentation/100067/0612/armclang-Command-line-Options/-mimplicit-it

In A32 code, the integrated assembler accepts all conditional instructions
without giving an error or warning. In T32 code, the integrated assembler
outputs an implicit IT block when there is a conditional instruction
without an enclosing IT block. The integrated assembler does not give an
error or warning about this.

Signed-off-by: chao an <anchao@xiaomi.com>

arch/arm/src/common/Toolchain.defs

@@ -80,6 +80,11 @@ ifeq ($(CONFIG_ARM_THUMB),y)
   # -mthumb option directly to the assembler by prefixing it with -Wa.
 
   ARCHOPTIMIZATION += -Wa,-mthumb
+
+  # Outputs an implicit IT block when there is a conditional instruction
+  # without an enclosing IT block.
+
+  ARCHOPTIMIZATION += -Wa,-mimplicit-it=always
 endif
 
 ifeq ($(CONFIG_UNWINDER_ARM),y)