[llvm] [X86] Add regcall4 attribute to make a specific function respect regcall ABIv4 (PR #69628)

[via llvm-commits]

yubingex007-a11y wrote:

there is question here should we really add a new calling conv CC_X86RegCall4 in CFE.
currently i let regcall4 attr reuse CC_X86RegCall instead of creating CC_X86RegCall4 . 

@mikerice1969 

https://github.com/llvm/llvm-project/pull/69628