[MC][ELF] Change SHT_LLVM_CALL_GRAPH_PROFILE relocations from SHT_RELA to SHT_REL

... even on targets preferring RELA. The section is only consumed by ld.lld
which can handle REL.

Follow-up to D104080 as I explained in the review. There are two advantages:

* The D104080 code only handles RELA, so arm/i386/mips32 etc may warn for -fprofile-use=/-fprofile-sample-use= usage.
* Decrease object file size for RELA targets

While here, change the relocation to relocate weights, instead of 0,1,2,3,..
I failed to catch the issue during review.