Update release notes for lld 7.

lld is a high-performance linker that supports ELF (Unix), COFF (Windows),

Mach-O (macOS), MinGW and WebAssembly. lld is command-line-compatible with GNU

linkers and Microsoft link.exe, and is significantly faster than these system

linkers and Microsoft link.exe, and is significantly faster than the system

default linkers.

lld 7 for ELF, COFF and MinGW are production-ready. lld/ELF can build the entire

FreeBSD/AMD64 and will be the default linker of the next version of the

operating system. lld/COFF is being used to build popular large programs such as

the Chrome web browser. lld/MinGW is being used by Firefox for their MinGW

builds. lld/MinGW still needs a sysroot specifically built for lld, with

llvm-dlltool, though. Mach-O and WebAssembly supports are still experimental.

lld 7 for ELF, COFF and MinGW are production-ready.

* lld/ELF can build the entire FreeBSD/{AMD64,ARMv7} and will be the default

  linker of the next version of the operating system.

* lld/COFF is being used to create official builds of large popular programs

  such as Chrome and Firefox.

* lld/MinGW is being used by Firefox for their MinGW builds. lld/MinGW still

  needs a sysroot specifically built for lld, with llvm-dlltool, though.

* lld/WebAssembly is used as the default (only) linker in Emscripten when using

  the upstream LLVM compiler.

* lld/Mach-O is still experimental.

Non-comprehensive list of changes in this release

=================================================

* Changed section layout to improve compatibility with link.exe.

* `/subsystem` inference is improved to cover more corner cases.

* Added the following flags: ``--color-diagnostics={always,never,auto}``,

  ``--no-color-diagnostics``, ``/brepro``, ``/debug:full``, ``/debug:ghash``,

  ``/guard:cf``, ``/guard:longjmp``, ``/guard:nolongjmp``, ``/integritycheck``,