Updated x86-64 psABI

Hi,

The x86-64 psABI has been updated to revision 248. Main changes are

1. Support Intel MPX.
2. Add a chapter for linker optimization.
3. Add 2 new relocations, R_X86_64_GOTPCRELX and R_X86_64_REX_GOTPCRELX.

MPX supported has been checked into GCC 5 and ld in binutils 2.25.
Linker optimization has been added to ld in binutils 2.25 and gold
in binutils 2.26. Gold and ld in binutils 2.26 supports new relocations.
Ld in binutils 2.26 can optimize new relocations.

The psABI PDF files are available at

The x86-64 psABI draft 2015-11-18 contains updated MPX spec.

I uploaded x86-64 psABI revision 249, which is equivalent to
the x86-64 psABI draft 2015-11-18, PDF file to