The issue was that the pause instruction was emitted through an asm
directive which was not correct for some mips platforms.

Simplified boost-fiber-exclude as a result.

Removed uClibc-ng math patch. It was not correct as it broke float and
long double support (std variants use function overloads). A different
solution was applied upstream. As it's quite annoying to backport, just
wait until a new release comes with that change. ARC as a platform is
barely supported anyways.

Swapped asio patch for the upstream submission, which is unfortunately, in
limbo.

Refreshed remaining patch.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

This commit updates Boost to version 1.71.0 and disables Boost.Context
 for arc and mips64 architectures, since either jump_fcontext or
 getcontext are undefined for those architectures.

It also fixes a bug were Boost.Fiber was not properly disabled for
mips32 and mips64 architectures.

Boost.Coroutine2 option was removed since it was redundant. By selecting
the Coroutine package, Coroutine2 is also installed.

Boost.Fiber has been disabled for target brcm47xx_generic and brcm47xx_legacy
due to misssing opcode support from instruction set.

Boost 1.71.0 brings a new header-only library
 - Boost.Variant2 [1]
   -> A never-valueless, strong guarantee implementation of
      std::variant, from Peter Dimov.

More info about Boost 1.71.0 can be found at the usual place [2].

[1]: https://www.boost.org/doc/libs/1_71_0/libs/variant2/doc/html/variant2.html
[2]: https://www.boost.org/users/history/version_1_71_0.html

Signed-off-by: Carlos Miguel Ferreira <carlosmf.pt@gmail.com>