toolchainRequirements recipe support

correct AArch64 detection and make consistent with ARM

speed up platform probe 50ms -> 5ms by avoiding phobos imports

Hide enums from public API.

Refactor determinePlatform, determineArchitecture, and determineCompiler from dub.platform into strings that can be pasted into generatePlatformProbeFile and remove unused imports.

Remove all trailing whitespace ...

> sed 's/[ \t]*$//' -i **/*.d

Improve diagnostics for invalid platform specifiers.

Move BuildPlatform to dub.platform and improve docs.

Various small fixes ...

- fix computeBuildID for --compiler=/my/path;
- dub compiled with ldc tried to invoke ldc as default binary instead of ldc2;
- dmd: Don't generate anything;
- gdc: Do not link, only generate an object;

Fix ARM FP version identifiers.

Implemented basic build platform detection and extended the possibilities for platform specific fields in package.json.