add support for x86_64-solaris2 platform

Summary:
this set of patches adds support for x86_64-solaris2 platform
Solaris is multi-lib platform which means it provides 32bit user-land together
with 32bit and 64bit libraries. The 32bit libraries are located in <somewhere>/lib
directories while 64bit libraries are located in <somewhere>/lib/64 directories.
This is why GHCi required the fix since otherwise it'll attempt to load
/usr/lib/libgmp.so which is 32bit library into 64bit binary process space (GHCi).
This of course fails with wrong ELFCLASS32 error message.
Another issue was that by default GNU C distributed with Solaris compiles
into 32bit binary. We need to enforce compilation to 64bit binary
by adding appropriate -m64 option.

Test Plan: already built on x86_64-solaris2

Reviewers: austin

Reviewed By: austin

Subscribers: phaskell, simonmar, relrod, carter

Differential Revision: https://phabricator.haskell.org/D68