toolchain.eclass: make toolchain_src_unpack() a default_src_unpack()

toolchain.eclass: make toolchain_src_unpack() a default_src_unpack()

Before tha change toolchain_src_unpack() did not unpack every SRC_URI
entry and instead relied on toolchain_src_unpack() to unpack only
needed tarballs. Any special handling was only needed for old USE=d
versions of gcc.

This change moves all source code mangling to src_prepare() and makes
toolchain_src_unpack() a default_src_unpack().

The only exception is ebuild users of 'GCC_A_FAKEIT' (dev-lang/gnat-gpl)
who have to use it to workaround non-standard behaviour of toolchain_src_unpack()

The change will allow dev-lang/gnat-gpl to migrate to default_src_unpack().

Bug: https://bugs.gentoo.org/706588
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>