[Sanitizers] Remove clang_rt.sancov_{begin,end} on Solaris

There's no point to manually create the __start___sancov_guards and __stop___sancov_guards
sections and labels on Solaris any longer.  They were originally introduced in
https://reviews.llvm.org/D40899 and https://reviews.llvm.org/D40903.

- The Solaris 11.4 ld supports creating them out of the box.
- We already unconditionally use Solaris 11.4 features like the ld -z gnu-version-script-compat option and fully working .preinit_array support in compiler-rt.
- The current files don't assemble on SPARC because the assembler syntax may be different between /bin/as and GNU as.

Tested on amd64-pc-solaris2.11.

Differential Revision: https://reviews.llvm.org/D63601

llvm-svn: 365399