Add c_long_double for interacting with C code.

Based on a recent newsgroup discussion, I think that it's clear that we
need an alias for C's long double, so here it is. I think that I got it
right for x86, and x86_64, but I have no idea what it should be for
other platforms.
Add `@nogc` attribute to `core.exception` functions.

The main change is `onOutOfMemoryError` & `onInvalidMemoryOperationError` are now `@nogc`.
fix Issue 13148 - ModuleInfo fields are unnecessary changed to const
Merge pull request #863 from jmdavis/long_double

Add c_long_double for interacting with C code.
Merge pull request #887 from denis-sh/add-`@nogc`-to-`core.exception`

Add `@nogc` attribute to `core.exception` functions.
Merge pull request #898 from 9rnsr/fix13148

[REG2.066a] Issue 13148 - ModuleInfo fields are unnecessary changed to const
Fix to c_long_double for GNU at Iain's request.
parial Issue 9449 - Static arrays of 128bit types segfault on initialization. Incorrect calling of memset128ii
Merge pull request #900 from jmdavis/c_long_double

Fix to c_long_double for GNU at Iain's request.
Merge pull request #901 from WalterBright/fix9449

[critical] partial Issue 9449 - Static arrays of 128bit types segfault on initializa...