[compiler-rt][builtins] Provide __clear_cache for SPARC

While working on https://reviews.llvm.org/D40900, two tests were failing since __clear_cache
aborted.  While libgcc's __clear_cache is just empty, this only happens because
gcc (in gcc/config/sparc/sparc.c (sparc32_initialize_trampoline, sparc64_initialize_trampoline))
emits flush insns directly.

The following patch mimics that.

Tested on sparcv9-sun-solaris2.11.

Differential Revision: https://reviews.llvm.org/D64496

llvm-svn: 366822