<osv/demangle.hh> - add missing include

The definitions of <osv/demangle.hh> require size_t. We should include
<stddef.h>, in case the user's file never included one of the other
header files which define size_t (I had such a case).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
execinfo: implement backtrace_symbols*

From Linux's <execinfo.h>, we already implemented backtrace() but
not backtrace_symbols() or backtrace_symbols_fd(). This patch
implements both. These functions are used by several applications,
including MySQL, so we should have them.

Fixes #493.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
trace: make symbol resolution more efficient

Fixes two issues:

  - when addr is in cache we call the delegate instead of
    returning from cache

  - we didn't cache the delegate's result so if we didn't have the
    symbol we always had to browse through our tables (but they don't
    change).

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>