Cut target.h into pieces.

[sandreenko]

Before:

After:

So now you don't need to memorize the order of the platforms in the target.t (note that the memorization did not help if there were more than 1 define for some platform, like on the screen).

That was a long time on my TODO list but I always wanted to do it like:

#ifdef PLATFORM_ARM
#define TIMEOUT_VALUE_IMPL 0
#define OUR_MAGIC_VALUE_IMPL 1
#elif PLATFORM_ARM64
#define TIMEOUT_VALUE_IMPL 2
#define OUR_MAGIC_VALUE_IMPL 3
#endif 

// a comment for TIMEOUT_VALUE
#define TIMEOUT_VALUE TIMEOUT_VALUE_IMPL

// a comment for OUR_MAGIC_VALUE
#define OUR_MAGIC_VALUE OUR_MAGIC_VALUE_IMPL

so we have comments in one place, but looks like I would not have time for it any time soon, so let's just cut it into pieces.

and small additional changes:
57d0580: C_ASSERT is not used there.

e3f5685: delete unused "REGNUM_MASK'.

e876ff0: delete redefines of "REGMASK_BITS".

[sandreenko]

PTAL @BruceForstall @dotnet/jit-contrib

[BruceForstall]

I'm in favor of this.

I think you should add the new headers to JIT_HEADERS in CMakeLists.txt

[sandreenko]

I think you should add the new headers to JIT_HEADERS in CMakeLists.txt

done