x86/vdso: Abstract out vdso system call internals

x86/vdso: Abstract out vdso system call internals

Abstract out the calling of true system calls from the vdso into
macros.

It has been a very long time since gcc did not allow %ebx or %ebp in
inline asm in 32-bit PIC mode; remove the corresponding hacks.

Remove the use of memory output constraints in gettimeofday.h in favor
of "memory" clobbers. The resulting code is identical for the current
use cases, as the system call is usually a terminal fallback anyway,
and it merely complicates the macroization.

This patch adds only a handful of more lines of code than it removes,
and in fact could be made substantially smaller by removing the macros
for the argument counts that aren't currently used, however, it seems
better to be general from the start.

[ v3: remove stray comment from prototyping; remove VDSO_SYSCALL6()
      since it would require special handling on 32 bits and is
      currently unused. (Uros Biszjak)

      Indent nested preprocessor directives. ]

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Uros Bizjak <ubizjak@gmail.com>
Link: https://patch.msgid.link/20251216212606.1325678-9-hpa@zytor.com