[llvm-exegesis/lib] Fix missing linkage to MCParser

Otherwise, shared-lib build fails with:

lib64/libLLVMExegesis.a(SnippetFile.cpp.o): In function `llvm::exegesis::readSnippets(llvm::exegesis::LLVMState const&, llvm::StringRef)':
SnippetFile.cpp:(.text._ZN4llvm8exegesis12readSnippetsERKNS0_9LLVMStateENS_9StringRefE+0x31f): undefined reference to `llvm::createMCAsmParser(llvm::SourceMgr&, llvm::MCContext&, llvm::MCStreamer&, llvm::MCAsmInfo const&, unsigned int)'
SnippetFile.cpp:(.text._ZN4llvm8exegesis12readSnippetsERKNS0_9LLVMStateENS_9StringRefE+0x41c): undefined reference to `llvm::MCAsmParser::setTargetParser(llvm::MCTargetAsmParser&)'
collect2: error: ld returned 1 exit status

llvm-svn: 373332