android: refine custom ucontext_t definitions.

android: refine custom ucontext_t definitions.

This is a forward-compatible change to avoid type/naming
conflicts when the Android platform/NDK will update its
<signal.h> header to properly define 'struct sigcontext',
'mcontext_t' and 'ucontext_t'.

In particular:

  - Do not define 'struct sigcontext.h' to avoid
    conflicts with the C library definition (which
    is different, see below).

  - Only provide custom ucontext_t declarations if
    the Android <signal.h> doesn't provide it. This can
    be tested with a macro check (__BIONIC_HAVE_UCONTEXT_T)

+ Use 'gettid()' on Android since it is available (at all
  API levels).

See http://code.google.com/p/android/issues/detail?id=34784

Review URL: https://chromiumcodereview.appspot.com/10829122
Patch from David Turner <digit@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12250 ce2b1a6d-e550-0410-aec6-3dcde31c8c00