use non-LTO IPO cflags, adjust ldflags for Identical Code Folding and…

[tpgxyz]

use non-LTO IPO cflags, adjust ldflags for Identical Code Folding and function Garbage Collector

Here is the long story short:

1. lld has a hardcoded options in a patch:
   --icf=safe
   --gc-sections
2. There are some software that intentionally does not use of {C,LD}FLAGS and this means above hardcoded flags can not be overrided by:

• exporting LDFLAGS or redifining %build_ldfags
• patching source files to and opposite options that disables above flags

3. lld --icf=safe implementation is diffrent from ld.bfd!

• This thread casts some light on it https://www.spinics.net/lists/linux-efi/msg19229.html
• hardcoded --icf=safe in lld breaks kernel, vmlinux is missing symbols
• inconsistent ORC unwind table entries in file: vmlinux in /scripts/sorttable vmlinux ClangBuiltLinux/linux#1341
• chromeos uses --icf=all and they claim this one is safe https://bugs.chromium.org/p/chromium/issues/detail?id=576197

4. Function Garbage Collector break things when upstream wants to keep specific input sections:

• see PE/COFF files (see vmlinux)
• This says -gc-sections is experimental https://sourceware.org/binutils/docs/ld/Options.html
• ld.lld: error in arch/arm64/kernel/vmlinux.lds ClangBuiltLinux/linux#1336

5. Using --gc-sections without -ffunction-sections -fdata-sections makes only half of the success

I did compile llvm without above hardcodes:
OpenMandrivaAssociation/llvm#12
then i build kernel-release for x86_64 and aarch64. Both kernels clang flovours uses lld as default linker.
x86_64: https://abf.openmandriva.org/build_lists/40029
aarch64: https://abf.openmandriva.org/build_lists/40016

[root@tpg-latitude5490 tpg]# uptime
 18:15:31 up 44 min,  4 users,  load average: 0,44, 0,39, 0,36
[root@tpg-latitude5490 tpg]# uname -a
Linux tpg-latitude5490 5.11.13-desktop-clang-1omv4050 #1 SMP PREEMPT Tue Apr 13 13:10:26 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
[root@tpg-latitude5490 tpg]# grep LLD /boot/config-5.11.13-desktop-clang-1omv4050 
CONFIG_LD_IS_LLD=y
CONFIG_LLD_VERSION=120000

[berolinux]

Something along those lines makes sense.
A few things:
There is no hardcode. We merely flip the defaults to save some space in a gazillion packages.
To override the --icf=safe default, you can use --icf=none.
To override the --gc-sections default, you can use --no-gc-sections.
It probably makes sense to keep current defaults and just override them where necessary, but having a well defined way to do that instead of having to fiddle with the flags manually is a good idea

[tpgxyz]

There is no hardcode. We merely flip the defaults to save some space in a gazillion packages.

Few of these defaults were moved to this PR so rpm macros may catch them.

To override the --icf=safe default, you can use --icf=none.
To override the --gc-sections default, you can use --no-gc-sections.

Some Makefies does not pass any LDFLAGS, see KBUILD in couple of places, so without patching Makefiles you are not able to pass --icf=none or --no-gc-sections. Please see ClangBuiltLinux bugs.
Anyways this PR pushes things forward, to explicitly set FLAGS instead of hardcoding these in some lld patch.