fix libtar symbols

Provided unique symbols instead of "common" names, like
"print_header" or "buffer". These symbols frequently clashed with
symbols from other software. Because libtar is a part of ZRT it is
linked to each and every binary.
Merge pull request #143 from YaroslavLitvinov/master

libtar symbols fixed
This fixed rudimentary symbols without prefix for libtar
Fixed bug caused to abort at exit

Call of postmain handler added to fix abnormal termination of zrt
program if /dev/alloc_report channel was present and also exporting
files feature was enabled in nvram file; Errors were occured while
collecting allocation info, particularly during backtrace call
which is not designed to be called out of bounds of main function.
Merge pull request #144 from YaroslavLitvinov/dev

Dev
zcall 'preinit' was added

mainly did it for compatibility with zerovm/glibc, which was
extended with __ZRT_HOST support.
zrt memory manager improvement

64bit build support is added, for cases when building it for host.
Merge pull request #145 from YaroslavLitvinov/dev

improvements and zrt host support