[ELF] Fix DT_MIPS_LOCAL_GOTNO value for thunks and linker scripts

[ELF] Fix DT_MIPS_LOCAL_GOTNO value for thunks and linker scripts

The MIPS GOT section has a number of local entries based on the number of pages
needed for output sections referenced by GOT page relocations. The number is
recorded in the DT_MIPS_LOCAL_GOTNO dynamic section tag. However, the dynamic tag
is added before assignAddresses has been called, meaning that any section size used
to calculate the value will not include size modifications caused by, for example,
linker scripts and thunks.

This change moves the calculation of DT_MIPS_LOCAL_GOTNO until writeTo, by which
time the output section sizes have been finalized.

Reviewers: ruiu, rafael

Differential Revision: https://reviews.llvm.org/D39493

llvm-svn: 318828