Meson calls `path/to/clang++ --version` to guess which build toolchain the
user has picked to build the source code. For the Qualcomm LLVM toolchain,
the output have an unusual output as shown below:
```
clang version 8.0.12
Snapdragon LLVM ARM Compiler 8.0.12 (based on llvm.org 7.0+)
Target: arm-unknown-linux-gnueabi
Thread model: posix
Repository: (ssh://git-hexagon-aus.qualcomm.com:...)
InstalledDir: /pkg/qct/software/llvm/release/arm/8.0.12/bin
```
Another unusual pattern is the output of `path/to/ld.qcld --version`:
```
ARM Linker from Snapdragon LLVM ARM Compiler Version 8.0.12
ARM Linker based on LLVM version: 8.0
```
The Meson logic is modified accordingly so that Meson can correctly
determine toolchain as "LLVM aarch64 cross-compiler on GNU/Linux64 OS".
This is the expected output of
`meson --native-file native_file.ini --cross-file cross_file.ini build/aarch64-debug/`:
```
...
C++ compiler for the host machine: ... (clang 8.0.12 "clang version 8.0.12")
C++ linker for the host machine: ... ld.lld 8.0.12
...
```
Put machine file and cmd line parsing in Environment
This creates a full set of option in environment that mirror those in
coredata, this mirroring of the coredata structure is convenient because
lookups int env (such as when initializing compilers) becomes a straight
dict lookup, with no list iteration. It also means that all of the
command line and machine files are read and stored in the correct order
before they're ever accessed, simplifying the logic of using them.
push all config-file settings into coredata.set_default_options
This puts all of them together, in the next patch they'll be pulled back
out, but it's convenient to start that refactor by moving them all
there, then moving them into env as a whole.
