[x86-64 psABI] Document STT_GNU_IFUNC and R_X86_64_IRELATIVE

Hi,

This patch for x86-64 psABI adds document for STT_GNU_IFUNC and
R_X86_64_IRELATIVE. It has been implemented on Linux/x86-64 for
more than a year.  Please add it to x86-64 psABI.

Thanks.

--
H.J.