Cygwin: signal: make context structures registers handling portable

 Cygwin: signal: make context structures registers handling portable
 
 This patch extracts macros from winsup/cygwin/exceptions.cc serving for portable
 register access to context structures into a separate local header
 winsup/cygwin/local_includes/register.h and implements their AArch64
 counterparts.
 
 Then, it adds AArch64 declaration of __mcontext structure based on
 mingw-w64-headers/include/winnt.h header to
 winsup/cygwin/include/cygwin/singal.h header.
 
 Then, it includes the registers.h header and uses the macros where applicable,
 namely at:
  - winsup/cygwin/exceptions.cc
  - winsup/cygwin/profil.c
  - winsup/cygwin/tread.cc
 
 The motivation is to make usage of the context structures portable without
 unnecessary #if defined(__x86_64__) while implementations of signal handling
 code will be developed later, e.g. implementation of makecontext.
 
 Signed-off-by: Radek Bartoň <radek.barton@microsoft.com>