It appears that LLVM 15 changed the ABI for _Float16 on x86-64 such that
values are now passed in `xmm0` instead of using integer registers.
Also enable this code for Linux.
rdar://104134160
It turns out that we can just use the Float16 to Float32 conversion and
let the compiler turn that into a long double for us, which means we
can take advantage of the F16C CPU instructions if they're present as well.
rdar://104134160
This replaces a number of `#include`-s like this:
```
#include "../../../stdlib/public/SwiftShims/Visibility.h"
```
with this:
```
#include "swift/shims/Visibility.h"
```
This is needed to allow SwiftCompilerSources to use C++ headers which include SwiftShims headers. Currently trying to do that results in errors:
```
swift/swift/include/swift/Demangling/../../../stdlib/public/SwiftShims/module.modulemap:1:8: error: redefinition of module 'SwiftShims'
module SwiftShims {
^
Builds.noindex/swift/swift/bootstrapping0/lib/swift/shims/module.modulemap:1:8: note: previously defined here
module SwiftShims {
^
```
This happens because the headers in both the source dir and the build dir refer to SwiftShims headers by relative path, and both the source root and the build root contain SwiftShims headers (which are equivalent, but since they are located in different dirs, Clang treats them as different modules).
When building for Windows x86, we do not have `__i686__` defined, but do
have `__i386__` defined. Ensure that the routines are included for the
x86 Windows target.
Android NDK <r21 does not provide the AEABI mandated `__aeabi_d2h`
functions in the compiler runtime. Add shims to ensure that the
functions are available for building the standard library.
