Apply -flax-vector-conversions flag to GCC

Some CMSIS-DSP code does not explicitly convert between vectors of
different numbers and types of elements. While some other compilers
default to implicitly converting vectors, gcc does not do this unless
explicitly enabled.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>

Source/configDsp.cmake

@@ -47,6 +47,12 @@ if (HELIUM OR MVEF OR SUPPORT)
    target_include_directories(${project} PRIVATE "${DSP}/PrivateInclude")
 endif()
 
+if (MVEI OR MVEF OR HELIUM)
+    # By default, GCC does not enable implicit conversion between vectors of different numbers or types of elements
+    # which is required by some code in CMSIS-DSP
+    target_compile_options(${project} PRIVATE $<$<STREQUAL:${CMAKE_C_COMPILER_ID},GNU>:-flax-vector-conversions>)
+endif()
+
 if (DISABLEFLOAT16)
     target_compile_definitions(${project} PRIVATE DISABLEFLOAT16) 
 endif()