Fix a bug in darwin's 32-bit X86 handling of evaluating fixups.

Where it would use a scattered relocation entry but falls back to a
normal relocation entry because the FixupOffset is more than 24-bits.

The bug is in the X86MachObjectWriter::RecordScatteredRelocation() where
it changes reference parameter FixedValue but then returns false to indicate
it did not create a scattered relocation entry.  The fix is simply to save the
original value of the parameter FixedValue at the start of the method and
restore it if we are returning false in that case.

rdar://15526046