Set has_asmparser in PowerPC/LLVMBuild.txt

PowerPC now has an asm parser (and has for many months now); indicate this in
PowerPC/LLVMBuild.txt.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197393 91177308-0d34-0410-b5e6-96231b3b80d8