Object: rename ARMV7 to ARMNT

The official specifications state the name to be ARMNT (as per the Microsoft
Portable Executable and Common Object Format Specification v8.3).
build: fix bleeding whitespace
build: use LLVM build routines

Check if the compiler actually supports the flags that are being added.
Previously, the compiler flags would be used improperly push the flags to the
compiler.  Particularly, on Darwin, the option would be pushed to the compiler
even if it does not support it.