[Mips] Move RelocationPass class member function definitions out of the
class.
[Mips] Move AHL addends calculation to the RelocationPass class.
[Mips] Add 'const' qualifier to the function.
[Mips] Factor out the code creates GOT entry atoms into the two
functions. The first one is for local GOT entries. The second one is for
global GOT entries.
[Mips] Handle R_MIPS_GOT16 relocation for external and local symbols in
a uniform way.
[Mips] Fix handling of R_MIPS_GOT16 relocation and building local part
of GOT.
 * Read addend for R_MIPS_GOT16 relocation.
 * Put only high 16 bits of symbol + addend into GOT entries for
   locally visible symbols.