[ARM64EC 4/?] Add LLVM support for varargs calling convention.

Part of patchset to add initial support for ARM64EC.

The ARM64EC calling convention is the same as ARM64 for non-varargs
functions, but for varargs, the convention is significantly different.
Basically, only x0-x3 registers are used for passing arguments, and x4
and x5 describe the address/size of the arguments passed in memory. (See
https://docs.microsoft.com/en-us/windows/uwp/porting/arm64ec-abi for
more details; see
https://docs.microsoft.com/en-us/cpp/build/x64-calling-convention for
the x64 calling convention rules, which this convention needs to match.)

Note that this currently doesn't handle i128 arguments correctly; as
noted in review, that's sort of complicated to handle, so I'm leaving it
for a followup.

Differential Revision: https://reviews.llvm.org/D125415

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