When lowering Call to external functions on x64, Subzero was not
correctly allocating 32 bytes for the shadow store. This became
apparent when trying to pass pointers to local variables to Call, in
which case, the address of the stack variable would be passed via
register. However, in the called external function, on x64, it's allowed
to use the first 32 bytes on the stack - the shadow store - as scratch
space. In my use-case, the called function ended up overwriting the
local stack variable that the callee passed a pointer to.

Another use-case where this error was apparent was in passing more than
four arguments to a Call, in which case, the callee expected the spilled
args to be present after the shadow store.

This fixes these problems by correctly allocating the 32 byte shadow
store when lowering Call on x64.

Note that this is similar to e81e8b3c
where I fixed the prolog to the generated entry point to take the shadow
store into account.

Bug: b/144688789
Change-Id: I9d7960bb9c520f5cc3c0ad885305ca5a3c06b5dc
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38473Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>