x86/elf: Use savesegment() for segment register reads in ELF core dump

x86/elf: Use savesegment() for segment register reads in ELF core dump

ELF_CORE_COPY_REGS() currently reads %ds, %es, %fs, and %gs using
inline assembly and manual zero-extension. This results in redundant
instructions like `mov %eax,%eax`.

Replace the inline assembly with the `savesegment()` helper, which
automatically zero-extends the value to the full register width,
eliminating unnecessary instructions.

For example, the %ds load sequence changes from:

  d03: 8c d8                 mov    %ds,%eax
  d05: 89 c0                 mov    %eax,%eax
  d07: 48 89 84 24 38 01 00  mov    %rax,0x138(%rsp)
  d0e: 00

to:

  ce8: 8c d8                 mov    %ds,%eax
  cea: 48 89 84 24 38 01 00  mov    %rax,0x138(%rsp)
  cf1: 00

thus eliminating the unnecessary zero-extending `mov %eax,%eax`.

No functional change intended.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://patch.msgid.link/20260330085938.67985-1-ubizjak@gmail.com