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'.
- 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