base/asm-common.h: Use new literal-pool stuff for ARM PIC macros.

base/asm-common.h: Use new literal-pool stuff for ARM PIC macros.

The support in GAS for the ARM literal-loading syntax is entirely
hopeless.

  * It unnecessarily refuses to allow expressions involving external
    symbols as literal values (e.g., `ldr r0, =_GLOBAL_OFFSET_TABLE_ -
    . - 12').  I've checked: if you modify GAS to remove the pointless
    check, then it produces the right result.

  * It doesn't use the enhanced expression evaluator which understands
    relocation annotations (so you can't say `ldr r0, =foo(GOT)').
    Indeed, this evaluator is internal to the handler for `.word' and
    friends.  Fixing this would be rather hard work, and involve
    uprating the literal-pool machinery quite a bit.

Instead, use the new subsection-based fake literal pool handling
machinery `_LIT' and `_ENDLIT' and some per-macro symbols with awful
names.

This also removes the magic knowledge which the previous version of the
`ldgot' had about the right PC offset, which would have been wrong for
Thumb code.