JIT: Switch GS cookie check temp reg to use r10 for tailcalls

[jakobbotsch]

rax was used before, but rax is used as an argument for the indirection cell for R2R tailcalls. r10 has a use for some pinvokes, but we do not tailcall for these.

Fix #120780

[Copilot]

Pull Request Overview

This PR fixes a register conflict issue in JIT compilation where the GS cookie check was using the rax register for tailcalls, but rax is also needed for R2R (Ready-to-Run) indirection cells during tailcalls. The fix changes the temporary register used for GS cookie checks in tailcall scenarios from rax to r10 on x64 platforms.

Key changes:

• Modified genGetGSCookieTempRegs to return r10 instead of rax for x64 tailcalls
• Added an assertion in AddFinalArgsAndDetermineABIInfo to verify that PInvoke functions with non-standard args are not tail-called

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/coreclr/jit/codegencommon.cpp	Changes GS cookie temp register from rax to r10 for x64 tailcalls and refactors conditional compilation directives
src/coreclr/jit/morph.cpp	Adds assertion and comment documenting that PInvoke functions are not tail-called, preventing conflict with r11

[jakobbotsch]

PTAL @dotnet/jit-contrib, another one of these, this time around the change that enabled tailcalls for functions with GS cookie checks on x64.