LDC 1.21.0-beta1

[kinke]

Glad to announce the first beta for an exciting LDC 1.21 release:

* Based on D 2.091.0; LLVM upgraded to v10.0.0-rc4.
* Experimental iOS/arm64 support.
* 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 and no more D 
`main()` requirement.
* Important AArch64 bugfix, especially for unoptimized code.

Full release log and downloads: 
https://github.com/ldc-developers/ldc/releases/tag/v1.21.0-beta1

Please help test, and thanks to all contributors!