[PowerPC] Optimize @ha/@l constructs

This patch adds support for having the assembler optimize fixups
to constructs like "symbol@ha" or "symbol@l" if "symbol" can be
resolved at assembler time.

This optimization is already present in the PPCMCExpr.cpp code
for handling PPC_HA16/PPC_LO16 target expressions.  However,
those target expression were used only on Darwin targets.

This patch changes target expression code so that they are
usable also with the GNU assembler (using the @ha / @l syntax
instead of the ha16() / lo16() syntax), and changes the
MCInst lowering code to generate those target expressions
where appropriate.

It also changes the asm parser to generate HA16/LO16 target
expressions when parsing assembler source that uses the
@ha / @l modifiers.  The effect is that now the above-
mentioned optimization automatically becomes available
for those situations too.