gnu-efi: Make setjmp.S portable to ARM

gnu-efi: Make setjmp.S portable to ARM

This patch fixes the following error:

  .../lib/arm/setjmp.S:18: Error: unrecognized symbol type ""
  .../lib/arm/setjmp.S:26: Error: unrecognized symbol type ""

The problem is the assembly syntax that is used is not portable to ARM,
where the '@' symbol is a comment:

  > Note on targets where the @ character is the start of a comment
  > (eg ARM) then another character is used instead. For example the
  > ARM port uses the % character.

(From https://sourceware.org/binutils/docs-2.25/as/Section.html#Section)

Signed-off-by: Wenzong Fan <wenzong.fan@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>