powerpc: add basic support for PLT relocations (#10402)

Commit a93ab43a
enabled support for proper PIC relocations from
assembler.

Commit adds support for relocations of type:
    R_PPC_REL16_HI
    R_PPC_REL16_HA
    R_PPC_REL16_LO
    R_PPC_PLTREL24

They are used only when GHC is built in
    DYNAMIC_GHC_PROGRAMS = NO
mode.

Verified by running the following test:

    // cat a.c
    #include <stdio.h>

    void ffi_a_hello (int i) {
        fprintf (stderr, "WEEEEEEEE: i=%d\n", i);
    }

    -- cat A.hs
    {-# LANGUAGE ForeignFunctionInterface #-}
    module A where

    import Foreign.C

    foreign import ccall "ffi_a_hello" a :: CInt -> IO ()

    # ghc -fPIC -c a.c -fforce-recomp
    # ghc -fPIC -c A.hs -fforce-recomp
    # ghc --interactive ./a.o A
    ...
    *A> a 42
    WEEEEEEEE: i=42
See gory details in Trac #10402.

Signed-off-by: Colin Watson <cjwatson@debian.org>
Signed-off-by: Sergei Trofimovich <siarheit@google.com>

Reviewed By: bgamari, austin

Differential Revision: https://phabricator.haskell.org/D996

GHC Trac Issues: #10402