Features added:
- nanoMIPS relocation resolving
- nanoMIPS eflags calculation
- .nanoMIPS.abiflags section creation
- nanoMIPS _gp symbol setup
- special .eh_frame handling in maybeReportUndefined
- R_NANOMIPS_PCHI20 renamed to R_NANOMIPS_PC_HI20

Added nanoMIPS specific linker relaxations/expansions:
- Transformations are conducted through a driver class
  NanoMipsTransformController. It scans through relocs of executable
  input sections and performs relaxations/expansions if possible or
  necessary
- New nanoMIPS specific options used in transformations added to lld
- New TableGen backend (NanoMipsTransformationPropertyEmitter.cpp) for
  emitting properties of instructions and relocations, transformation
  templates used in linker transformations

Changed parsing of the linker script, as some nanoMIPS linker scripts
require different set of rules for parsing:
- Dot operator can be used as section address expression, and its
  value should be determined by the latest value of dot, not the
  current position in the memory region
- Alignment for output section is not ignored for nanoMIPS if output
  section has got address expression
- SHT_NOBITS sections don't occupy load addresses
- OVERLAY sections can have a load region and memory region and
  assignment or byte commands in them.

Changed the way .eh_frame parsing is done for nanoMIPS, as they may
contain relocations referring to the size of the CIEs/FDEs. lld reads
.eh_frame sections using these sizes so it is necessary that the size
value is available at the time of .eh_frame section parsing.

It seems like integrated assembler for nanoMIPS is not resolving this
relocation at assembly time, but the gnu's assembler for nanoMIPS is
resolving it.

Linker relaxation may change relocations (offsets and types). However,
when --emit-relocs is used, relocations are simply copied from the input
section causing a mismatch with the corresponding (relaxed) code
section.

This patch fixes this as follows: for non-relocatable RISC-V binaries,
`InputSection::copyRelocations` reads relocations from the relocated
section's `relocations` array (since this gets updated by the relaxation
code). For all other cases, relocations are read from the input section
directly as before.

In order to reuse as much code as possible, and to keep the diff small,
the original `InputSection::copyRelocations` is changed to accept the
relocations as a range of `Relocation` objects. This means that, in the
general case when reading from the input section, raw relocations need
to be converted to `Relocation`s first, which introduces quite a bit of
boiler plate. It also means there's a slight code size increase due to
the extra instantiations of `copyRelocations` (for both range types).

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D159082

Similar to RISC-V relaxations, nanoMIPS transformations change
relocations and they need to be updated before writing them down to
output. One difference is that nanoMIPS can add new relocations, not
just change them like RISC-V, so during linker transformations size of
reloc sections need to be updated if --emit-relocs option is on.