Sergei Trofimovich authored Mar 29, 2017 and Ben Gamari committed Mar 29, 2017

The #13491 manifests best when we try to crosscompile
from 32-bit (i386-linux) to 64-bit (powerpc64-linux)
system:
    ./configure --target=powerpc64-unknown-linux-gnu

The build fails at assembly time:
  "inplace/bin/ghc-stage1" ...  -c rts/StgStartup.cmm
    /tmp/ghc19687_0/ghc_4.s: Assembler messages:

    /tmp/ghc19687_0/ghc_4.s:11:0: error:
         Error: unknown pseudo-op: `.l'
       |
    11 | .L<\x00>4:
       | ^

That happens because UNIQUE_BITS is defined in terms
of WORD_SIZE_IN_BITS macro:
    #define UNIQUE_BITS (WORD_SIZE_IN_BITS - 8)

WORD_SIZE_IN_BITS is 64 bits (equals to target value)
while ghc-stage1 is still running on i386-linux

The fix is to stop relying on target macros and use
host's 'sizeof (HsInt)' and 'finiteBitSize' way to
determine unique layout.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

Test Plan: build i386-to-powerpc64 crosscompiler

Reviewers: rwbarton, austin, bgamari

Reviewed By: bgamari

Subscribers: RyanGlScott, thomie

Differential Revision: https://phabricator.haskell.org/D3397