x86/tdx: Do not corrupt frame-pointer in __tdx_hypercall()

If compiled with CONFIG_FRAME_POINTER=y, objtool is not happy that
__tdx_hypercall() messes up RBP:

  objtool: __tdx_hypercall+0x7f: return with modified stack frame

Rework the function to store TDX_HCALL_ flags on stack instead of RBP.

[ dhansen: minor changelog tweaks ]

Fixes: c30c4b25 ("x86/tdx: Refactor __tdx_hypercall() to allow pass down more arguments")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/202301290255.buUBs99R-lkp@intel.com
Link: https://lore.kernel.org/all/20230130135354.27674-1-kirill.shutemov%40linux.intel.com