kconfig: Make CONIG_FP_SOFTABI generate floating point instructions

This re-applies a patch that was merged in pr zephyrproject-rtos#6989, but then
accidentally reverted in a bad merge conflict resolution in pr zephyrproject-rtos#6842.

The actual behaviour of CONIG_FP_SOFTABI has not been matching the
documenation. The Kconfig documentation states that floating point
instructions should be generated, but this Kconfig option has been
turning off floating point instructions instead.

This commit causes floating point instructions to be generated when
CONFIG_FP_SOFTABI is enabled, as was originally intended and
documented.

This commit can cause regressions if users have been relying on the
actual behaviour instead of the documented behaviour.

Kconfig documentation:

config FP_SOFTABI
	help
	  This option selects the Floating point ABI in which hardware
	  floating point instructions are generated but soft-float calling
	  conventions.

GCC documentation:

Specifying ‘soft’ causes GCC to generate output containing library
calls for floating-point operations. ‘softfp’ allows the generation of
code using hardware floating-point instructions, but still uses the
soft-float calling conventions. ‘hard’ allows generation of
floating-point instructions and uses FPU-specific calling conventions.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>

Author: SebastianBoe

cmake/compiler/gcc.cmake

@@ -93,7 +93,7 @@ else()
     if(CONFIG_FLOAT)
       list(APPEND TOOLCHAIN_C_FLAGS -mfpu=${FPU_FOR_${GCC_M_CPU}})
       if    (CONFIG_FP_SOFTABI)
-        list(APPEND TOOLCHAIN_C_FLAGS -mfloat-abi=soft)
+        list(APPEND TOOLCHAIN_C_FLAGS -mfloat-abi=softfp)
       elseif(CONFIG_FP_HARDABI)
         list(APPEND TOOLCHAIN_C_FLAGS -mfloat-abi=hard)
       endif()