libucontext: Add package

libucontext: Add package

Description (from libucontext github page):
libucontext (https://https://github.com/kaniini/libucontext) is a
library which provides the ucontext.h C API. Unlike other
implementations, it faithfully follows the kernel process ABI when
doing context swaps. libucontext is used on almost all musl
distributions to provide the legacy ucontext.h API.

This package is meant as a development package. There is no need
to install a package on the router if an application or library
is linked against the static libraries. Though, shared libraries
are provided also.

It is used to link libraries/applications against it which need the
system calls

* makecontext
* swapcontext
* getcontext
* setcontext

E.g. the asynchronous API of libmariadb (c-connector) uses this
system calls. Because libmusl didn't provide that system calls this
synchronous API is currently (without libucontexe) not working - it
segfaults.

Co-developed-by: Tianling Shen <cnsztl@immortalwrt.org>
Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>
Signed-off-by: Volker Christian <me@vchrist.at>