[mips] Update release notes for 3.7.0 release.

Changes to the MIPS Target

--------------------------

 During this release ...

During this release the MIPS target has:

* Added support for MIPS32R3, MIPS32R5, MIPS32R3, MIPS32R5, and microMIPS32.

* Added support for dynamic stack realignment. This of particular importance to

  MSA on 32-bit subtargets since vectors always exceed the stack alignment on

  the O32 ABI.

* Added support for compiler-rt including:

  * Support for the Address, and Undefined Behaviour Sanitizers for all MIPS

    subtargets.

  * Support for the Data Flow, and Memory Sanitizer for 64-bit subtargets.

  * Support for the Profiler for all MIPS subtargets.

* Added support for libcxx, libcxxabi, libunwind

* Improved inline assembly support such that memory constraints may now make use

  of the appropriate address offsets available to the instructions. Also, added

  support for the ``ZC`` constraint.

* Added support for 128-bit integers on 64-bit subtargets and 16-bit floating

  point conversions on all subtargets.

* Added support for read-only ``.eh_frame`` sections by storing type information

  indirectly.

* Added support for MCJIT on all 64-bit subtargets as well as MIPS32R6.

* Various bug fixes. Including the following notable fixes:

  * Fixed 'jumpy' debug line info around calls where calculation of the address

    of the function would inappropriately change the line number.

  * Fixed missing ``__mips_isa_rev`` macro on the MIPS32R6 and MIPS32R6

    subtargets.

  * Fixed representation of NaN when targeting systems using traditional

    encodings. Traditionally, MIPS has used NaN encodings that were compatible

    with IEEE754-1985 but would later be found incompatible with IEEE754-2008.

  * Fixed multiple segfaults and assertions in the disassembler when

    disassembling instructions that have memory operands.

  * Fixed multiple cases of suboptimal code generation involving $zero.

  * Fixed code generation of 128-bit shifts on 64-bit subtargets.

  * Prevented the delay slot filler from filling call delay slots with

    instructions that modify or use $ra.

  * Fixed some remaining N32/N64 calling convention bugs when using small

    structures on big-endian subtargets.

  * Fixed missing sign-extensions that are required by the N32/N64 calling

    convention when generating calls to library functions with 32-bit

    parameters.

  * Corrected the ``int64_t`` typedef to be ``long`` for N64.

  * ``-mno-odd-spreg`` is now honoured for vector insertion/extraction

    operations when using -mmsa.

  * Fixed vector insertion and extraction for MSA on 64-bit subtargets.

  * Corrected the representation of member function pointers. This makes them

    usable on microMIPS subtargets.

Changes to the PowerPC Target

-----------------------------