Adjust test golden output for Darwin

which has different regex error messages to glibc.
Add make rules to build .s files

which is useful when debugging assembly level problems e.g. for
intercepts.
Added a no-call trace mechanism

which formats either strings or hex numbers to a log file without doing
any calls or otherwise messing with the stack - because calling printf()
at certain points in the intercept code is downright dangerous.
Replace intercept debugging prints with trace()s

which can be enabled at build time with a configure option instead of by
editing the code.
Added a test program for OSX coredumps

...because they're weird and un-UNIXy.
Handle ASLR when mocking functions by name
Test intercepting oddball libc function prologs in Darwin
Re-implement intercepts for Darwin

This turned out to be rather more surgery than I expected.

The hack I was using on Linux to enable setcontext() to return back into
a label in the same function that called setcontext(), turned out not to
work with LLVM/clang, which (once the trace code was added) started
inserting instructions that trashed %rax between the label and the first
__asm__ statement after the label.  This caused lossage of the original
function's return value.

The fix turned out to be fairly complex.  The 2nd half of
intercept_tramp() is now a separate function __np_intercept_after().
The setcontext() call sets up a return address which points at a small
manually-coded assembler trampoline, which saves %rax, discovers the
original stack frame set up by intercept_tramp(), sets up a call to
__np_intercept_after(), and then returns to intercept_tramp()s original
caller.  Actually we need two such routines, one for the "push
%rbp" path and one for the oddball path.

This hasn't been tested with Valgrind yet.
Add a macro NP_USED

which is useful when the compiler aggressively optimises out functions
declared with static linkage and never called directly from C code, like
all the test functions in the meta-tests.

This is an API change.
Define NP_CLOCK_REALTIME for Linux
Include warnings when normalising test logs