[vm/ffi] Support varargs

This CL introduces `VarArgs` to `NativeFunction` signatures. The
`VarArgs` type takes a single type argument. This type argument is a
subtype of `NativeType` if there is a single variadic argument, and a
record with native types if there are multiple variadic arguments.
For example:
`NativeFunction<Void Function(Pointer<Char>, VarArgs<(Int32,Int32)>)>`
for calling refering to a `printf` binding with two `int32_t` arguments
passed as variadic arguments.

The logic of the native calling conventions are detailed in
https://dart-review.googlesource.com/c/sdk/+/278342.
Here we explain how this influences the FFI pipeline.

First, now that `VarArgs` is part of signatures, we have to unwrap
that when with the C types in the CFE transform and checking (analyzer
is in a separate CL), and also in the marshaller when looking up the
C type of arguments.

Second, we have to deal with `BothNativeLocations`. On windows x64,
floating point arguments must be passed both in FPU _and_ CPU
registers. For FFI calls, we solve this in the argument moves by just
copying to both locations. For FFI callbacks, we just take the FPU
register location (which avoids an extra bitcast).

Third, on System-V, we have to pass an upper bound of the number of
XMM registers used in AL. This means we instead RAX, we use R13 for the
target address. For variadic calls, we always pass 8 in AL as the valid
upper bound. We could consider passing the actual number of XMM
registers used.
We keep using RAX as default register for the function address on non-
variadic calls, because changing to R13 (the first free) register
creates more spilling in leaf calls. R13 is callee-saved while RAX is
not, so using R13 instead of RAX causes us to have to spill the value
from RAX on leaf calls.

Fourth, on both x64 and RISC-V, we pass floats in integer locations.
`EmitNativeMove` has been modified to deal with this, so that we do not
have to insert more `BitCastInstr`s.

The tests are generated by a test generator: `tests/ffi/generator/`.

The formatter doesn't support records yet, so the tests are not properly
formatted.
Bug: https://github.com/dart-lang/sdk/issues/50798

TEST=tests/ffi/*_varargs_*

Closes: https://github.com/dart-lang/sdk/issues/38578
Closes: https://github.com/dart-lang/sdk/issues/49460
Closes: https://github.com/dart-lang/sdk/issues/50858

Change-Id: I6a6296fe972527f8a54ac75a630131769e3cc540
Cq-Include-Trybots: luci.dart.try:vm-kernel-reload-rollback-linux-debug-x64-try,vm-kernel-reload-linux-debug-x64-try,vm-kernel-linux-debug-ia32-try,vm-kernel-nnbd-linux-debug-ia32-try,vm-kernel-win-debug-ia32-try,vm-kernel-linux-debug-x64-try,vm-kernel-mac-debug-x64-try,vm-kernel-win-debug-x64-try,vm-kernel-nnbd-win-release-ia32-try,vm-kernel-nnbd-win-debug-x64-try,vm-ffi-android-debug-arm-try,vm-ffi-android-debug-arm64c-try,vm-kernel-precomp-android-release-arm64c-try,vm-kernel-precomp-android-release-arm_x64-try,vm-precomp-ffi-qemu-linux-release-arm-try,vm-precomp-ffi-qemu-linux-release-riscv64-try,vm-kernel-asan-linux-release-x64-try,vm-kernel-precomp-asan-linux-release-x64-try,vm-kernel-msan-linux-release-x64-try,vm-kernel-precomp-msan-linux-release-x64-try,app-kernel-linux-debug-x64-try,vm-kernel-mac-release-arm64-try,vm-kernel-nnbd-mac-debug-arm64-try,vm-kernel-nnbd-mac-debug-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/276921
Reviewed-by: Devon Carew <devoncarew@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

https://dart.googlesource.com/sdk/+/6ef57b86c1ba78d291726b273e345a2df33f16b6