[libc] Bring libc in-tree

Moving libc from its separate repo into this one, minor resulting build
fixes, and a hacky way to add -I for libc headers in builds.

scripts/templates/target.host.j2

@@ -24,6 +24,7 @@ ccflags = $ccflags $
     -D__ELF__ $
     -D__JSIX__ $
     -isystem${srcroot}/sysroot/include $
+    -isystem${srcroot}/src/libraries/libc/include $
     --sysroot="${srcroot}/sysroot"
 
 cxxflags = $cxxflags $