Add support for ppc64le (PowerPC 64bit little endian)

Submitted by Michel Normand

Link to original bug (#745433)

Description

with orc 0.4.22 the make check is failing

FAIL: test-limits

investigation identified two related required changes:

• m4/as-host-defines.m4
  to define HAVE_POWERPC for ppc64le.
• orc/orcprogram-altivec.c
  to add support of ppc64le (that is PowerPC ABI V2)
  at least epilog/prolog differs, but need to check all file.