Remove Emscripten support

lib/Target/LLVMBuild.txt

 ;===- ./lib/Target/LLVMBuild.txt -------------------------------*- Conf -*--===;
 ;
 ;                     The LLVM Compiler Infrastructure
 ;
 ; This file is distributed under the University of Illinois Open Source
 ; License. See LICENSE.TXT for details.
 ;
 ;===------------------------------------------------------------------------===;
 ;
 ; This is an LLVMBuild description file for the components in this subdirectory.
 ;
 ; For more information on the LLVMBuild system, please see:
 ;
 ;   http://llvm.org/docs/LLVMBuild.html
 ;
 ;===------------------------------------------------------------------------===;
 
 [common]
-subdirectories = JSBackend ARM AArch64 BPF CppBackend Hexagon MSP430 NVPTX Mips PowerPC R600 Sparc SystemZ X86 XCore
+subdirectories = ARM AArch64 BPF CppBackend Hexagon MSP430 NVPTX Mips PowerPC R600 Sparc SystemZ X86 XCore
 
 ; This is a special group whose required libraries are extended (by llvm-build)
 ; with the best execution engine (the native JIT, if available, or the
 ; interpreter).
 [component_0]
 type = LibraryGroup
 name = Engine
 parent = Libraries
 
 ; This is a special group whose required libraries are extended (by llvm-build)
@@ skipping 17 matching lines @@
 parent = Libraries
 required_libraries = Analysis Core MC Support
 
 ; This is a special group whose required libraries are extended (by llvm-build)
 ; with every built target, which makes it easy for tools to include every
 ; target.
 [component_4]
 type = LibraryGroup
 name = all-targets
 parent = Libraries