libxml2: better patch for pthread issue

their intention was to weak link the pthread functions,
however this is broken for statical linkage anyway.
the actual bug causing the build error is due to prevention of
macro expansion  (one of musl's pthread_ funcs is a macro) in their
declaration.
libxml2: do not hide build output
musl: add bugfix for memory corruption through memalign

this was caught by the new error detection code in malloc, and caused
some libxml2 users to segfault: gegl and shared-mime-info.
it looked like a heisenbug since libxml2 makes usage of rand(), so
it was very hard to track it down...