cmake: define BOOST_USE_UCONTEXT tree-wide under ASan

cmake: define BOOST_USE_UCONTEXT tree-wide under ASan

Under WITH_ASAN Boost.Context is built ucontext-only, so consumers that
include its headers without linking Boost::context (e.g. libosd) were
still built for the fcontext backend and broke the link:

    mold: error: undefined symbol: boost::context::detail::make_fcontext

Define the backend tree-wide so every consumer agrees on it.

riscv64's ASan runtime mis-handles makecontext/swapcontext, so the
ucontext fiber backend reports false-positive heap-buffer-overflows on
fiber switch that can't be suppressed. So exclude it.

Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>