fix typo(s)

Remove leading hyphen in platform specs ...

It doesn't serve any purpose.

Refactor checks for platform is Windows into a method on BuildPlatform

Add architecture x86_omf for windows-dmd

Add support for iOS/tvOS/watchOS

Add playstation4 to platforms. (#1900)

Enable specifying full target triples as --arch for ldc2 ...

By treating all 'architectures' containing a `-` as `-mtriple` values.
E.g., `dub --arch=wasm32-unknown-unknown-wasm` to cross-compile to
WebAssembly. This dub switch is already used when probing the compiler,
so the build platform detection works as expected.

Also add `wasm` as detected platform and use the .wasm executable file
extension (for ldc2).

This fixes #1749 when moving the `-mtriple` option from the dub config
file to the dub command line.

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.