Commit the platform definition and build script work necessary to
cross-compile for arm64_32.
arm64_32 is a variant of AARCH64 that supports an ILP32 architecture.
This replaces the `LLVM_ATTRIBUTE_ALWAYS_INLINE` with `SWIFT_INLINE`
which is equivalent but namespaced to Swift instead. This reduces the
unnecessary reliance on LLVMSupport.
Silence warnings about deleted defaulted constructors due to the
non-trivial constructor for the atomic type. Guard a conditionally used
function with the appropriate guard.
Previously we had a single mask for all x86-64 targets which included both the top and bottom bits. This accommodated simulators, which use the top bit, while macOS uses the bottom bit, but reserved one bit more than necessary on each. This change breaks out x86-64 simulators from non-simulators and reserves only the one bit used on each.
rdar://problem/34805348 rdar://problem/29765919
Update the instances of checks for architectures to be more broad for different
spellings of the architecture macro. Certain targets use `_M_ARM` and others
use `__arm__`. Similarly, arm64/aarch64 has `_M_ARM64`, `__arm64__` and
`__aarch64__` as spellings. This just mechanically goes through and encodes the
various spellings.
Take the opportunity to replace some raw checks with `defined` checks which
avoids a pedantic warning due to the undefined macro when performing the check
as the preprocessor may warn about an undefined condition evaluating to `0`.
