toolchain updates for flight pro 8096 kyro cross compilation

[plusk01]

These changes allow the linux_app.cmake macros to correctly build binaries and shared objects. The preconditions for using this toolchain with the sfpro is that the Intrinsyc instructions for setting up the cross-compilation environment and the DSPAL/SLPI dev env have been completed.

Note: by removing the search path to sdsprpc.so (${HEXAGON_SDK_ROOT}/${SDKLIB}/common/remote/ship/UbuntuARM_${RELEASE}), we allow the linker to find the correct sdsprpc.so (correct in the sense that it is a soft-float abi). Without this change, the following error about VFP register arguments occurs:

/home/dspal/Qualcomm/cross_compiler/sysroots/x86_64-oesdk-linux/usr/libexec/arm-oe-linux-gnueabi/gcc/arm-oe-linux-gnueabi/4.9.3/real-ld: error: \
/home/dspal/Qualcomm/Hexagon_SDK/3.1/libs/common/remote/ship/UbuntuARM_Debug/libsdsprpc.so uses VFP register arguments, output does not

The linker is complaining that the compiler being used (from the Intrinsyc cross-compiler toolchain) does not have a hard-float abi, but the sdsprpc.so in the Hexagon SDK does [1]. This is verified using readelf -A libsdsprpc.so:

$ readelf -A Qualcomm/Hexagon_SDK/3.1/libs/common/remote/ship/UbuntuARM_Debug/libadsprpc.so 
Attribute Section: aeabi
File Attributes
  Tag_CPU_name: "7-A"
  Tag_CPU_arch: v7
  Tag_CPU_arch_profile: Application
  Tag_ARM_ISA_use: Yes
  Tag_THUMB_ISA_use: Thumb-2
  Tag_FP_arch: VFPv3
  Tag_Advanced_SIMD_arch: NEONv1
  Tag_ABI_PCS_wchar_t: 4
  Tag_ABI_FP_rounding: Needed
  Tag_ABI_FP_denormal: Needed
  Tag_ABI_FP_exceptions: Needed
  Tag_ABI_FP_number_model: IEEE 754
  Tag_ABI_align_needed: 8-byte
  Tag_ABI_align_preserved: 8-byte, except leaf SP
  Tag_ABI_enum_size: int
  Tag_ABI_HardFP_use: Deprecated
  Tag_ABI_VFP_args: VFP registers                  # <--- indicates hard-float abi
  Tag_ABI_optimization_goals: Aggressive Debug
  Tag_CPU_unaligned_access: v6

However, when we check the lib in the target sysroot:

$ readelf -A Qualcomm/cross_compiler/sysroots/aarch64-oe-linux/usr/lib/libsdsprpc.so
Attribute Section: aeabi
File Attributes
  Tag_CPU_name: "7-A"
  Tag_CPU_arch: v7
  Tag_CPU_arch_profile: Application
  Tag_ARM_ISA_use: Yes
  Tag_THUMB_ISA_use: Thumb-2
  Tag_FP_arch: VFPv3
  Tag_Advanced_SIMD_arch: NEONv1
  Tag_ABI_PCS_wchar_t: 4
  Tag_ABI_FP_rounding: Needed
  Tag_ABI_FP_denormal: Needed
  Tag_ABI_FP_exceptions: Needed
  Tag_ABI_FP_number_model: IEEE 754
  Tag_ABI_align_needed: 8-byte
  Tag_ABI_enum_size: int
  Tag_ABI_HardFP_use: Deprecated
  Tag_CPU_unaligned_access: v6

That line is not present because a soft-float abi is being used.

It seems that using the qurt_bundle CMake macro does not expose this problem because of the add_custom_target stanza:

add_custom_target(${QURT_BUNDLE_APP_NAME}_app ALL
    COMMAND ${QURT_BUNDLE_APPS_COMPILER}  ${${QURT_BUNDLE_APP_NAME}_INCLUDE_DIRS} ${QAIC_INCLUDE_DIRS} -o ${CMAKE_CURRENT_BINARY_DIR}/${QURT_BUNDLE_APP_NAME} ${QURT_BUNDLE_APPS_SOURCES} ${HEXAGON_SDK_ROOT}/${SDKLIB}/common/rpcmem/src/rpcmem.c "${CMAKE_CURRENT_BINARY_DIR}/${QURT_BUNDLE_IDL_NAME}_stub.c" ${FASTRPC_ARM_LIBS}
    DEPENDS generate_${QURT_BUNDLE_IDL_NAME}_stubs
    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
)

where APPS_COMPILER tends to be passed in as Qualcomm/ARM_Tools/gcc-4.9-2014.11/bin/arm-linux-gnueabihf-gcc, which has a hard-float abi [2].

---

[1]: This is likely related to the fact that Snapdragon 820 has a Kyro CPU, based on ARMv8-A (64 bit), but for some reason the yocto linux shipped is 32-bit.
[2]: Maybe this is actually itself a problem? Why do we have both soft-float and hard-float abi compilers? Why is the shipped OS 32-bit?