LDC 1.21.0

[kinke]

Glad to announce an exciting LDC 1.21 release - some highlights:

* Based on D 2.091.1+; LLVM upgraded to v10.0.0.
* Experimental iOS/arm64 support - all druntime/Phobos unittests 
pass, thanks Jacob! The prebuilt macOS package supports 
cross-compilation out of the box, just add 
`-mtriple=arm64-apple-ios12.0` (ldc2/ldmd2) or 
`--arch=arm64-apple-ios12.0` (dub).
* Initial support for GCC/GDC-style inline assembly syntax, 
primarily for portability across GDC and LDC (and GCC-style C).
* Android improvements, incl. an important fix for x86 
architectures, usability of the ld.gold linker, no more D main() 
requirement and reduced .so sizes.
* Important AArch64 bugfix, especially for unoptimized code.

Full release log and downloads: 
https://github.com/ldc-developers/ldc/releases/tag/v1.21.0

Thanks to all contributors!