Patches for LTO in the clang toolchain

[markyang92]

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This is a patch for LTO support in clang within yoe-distro.
You can test LTO in yoe-distro on an rpi4-64 environment with the following settings.

# yoe-distro/conf/local.conf
require conf/distro/include/lto.inc
DISTRO_FEATURES:append = " lto"
# or thin-lto
# DISTRO_FEATURES:append = " lto thin-lto"

I performed a build test with the following command.

$ bitbake yoe-simple-image

• The most severe problem currently seems to be recipes that force the bfd linker while using the clang toolchain.
  In yoe-simple-image, only bluez5, systemd-boot, and zlib are forced to use the bfd linker while using the clang Toolchain.
  Whether it's thin-lto or lto, when using the clang toolchain, the lld linker is recommended.
  If using the bfd linker, the LLVMgold.so plugin is required. This requires the gold linker.

However, the gold linker has been deprecated.

For now, the clang+bfd recipe has been avoided by setting LTO = "", but further consideration is needed.

[markyang92]

If we are using the bfd linker with Clang and want to use LTO functionality, the LLVMgold.so plugin must be supported.
It's not that the gold linker is required, but rather a plugin supported by LLVM.

To install this in llvm_git.bb,

EXTRA_OECMAKE:append:pn-llvm = " -DLLVM_BINUTILS_INCDIR=${STAGING_INCDIR}"
DEPENDS:append:pn-llvm = " binutils"

was added, and LTO = "" for zlib was disabled.

However, the following result is output:

yoe-distro/build/tmp/work/cortexa72-yoe-linux/zlib/1.3.1/recipe-sysroot-native/usr/bin/aarch64-yoe-linux/../lib/LLVMgold.so: error loading plugin

This is strange. It's looking for the path /usr/bin/lib/LLVMgold.so instead of /usr/lib/LLVMgold.so.

I think it's because cross-clang copies the location of clang. (ac290a5)

If I symlink clang in clang-cross again, it looks for the plugin at /usr/lib/LLVMgold.so.

I added -B <sysroot-native>/usr/lib or -Wl,-plugin,<sysroot-native>/usr/lib/LLVMgold.so, but it's the same...

It seems like clang is one level deeper and can't find the plugin correctly. Do you have any good ideas?

[kraj]

LGTM