rts/linker/ElfTypes.h: restore powerps (and others) support

Authored by trofi on May 14 2017, 2:33 PM.

Description

rts/linker/ElfTypes.h: restore powerps (and others) support

GHC build fails for powerpc-unknown-linux-gnu
and hppa-unknown-linux-gnu targets as:

rts_dist_HC rts/dist/build/RtsStartup.o
  rts/linker/ElfTypes.h:23:4: error:
       error: #error "Unsupported arch!"

Before the change code tried to whitelist architectures
and classify them into ELF32/ELF64. It does not work
for UNREG arches like 'hppa', 'sparc64', 'm68k', 'mips'.

It is nuanced for things like mips64 and x86_64:

'mips64-unknown-linux-gnu-gcc -mabi=64' is ELFCLASS64
'mips64-unknown-linux-gnu-gcc'          is ELFCLASS32
'x86_64-pc-linux-gnu-gcc'               is ELFCLASS64
'x86_64-pc-linux-gnu-gcc -mx32'         is ELFCLASS32

Here it's not enough to know HOST_ARCH. We really need to
know ABI.

The change uses 'LP64' as a proxy for ELFCLASS64.

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

Reviewers: angerman, simonmar, austin, bgamari, erikd

Reviewed By: angerman, bgamari, erikd

Subscribers: rwbarton, thomie

GHC Trac Issues: Trac #13696

Differential Revision: https://phabricator.haskell.org/D3583