Enable 7.6 build on all platforms

Patch is based on ab90767b with cleanup:

* don't have sys/reg.h so enclosing with HAVE_* macro
* appropriate type casts added where build producing warning
* macro conflicts between GDB elf/common.h and Bionic linux/elf.h
resolved differently than it was in original patch. See comments
inlined.
* add prototype for pread64(), see 999577bc

Change-Id: I66054c555ae251fee171a2b085417093b0c2fe16
Signed-off-by: Pavel Chupin <pavel.v.chupin@intel.com>