linux: Consolidate brk implementation

It removes all the arch-specific assembly implementation.  The
outliers are alpha, where its kernel ABI explict return -ENOMEM
in case of failure; and i686, where it can't use
"call *%gs:SYSINFO_OFFSET" during statup in static PIE.

Also some ABIs exports an additional ___brk_addr symbol and to
handle it an internal HAVE_INTERNAL_BRK_ADDR_SYMBOL is added.

Checked on x86_64-linux-gnu, i686-linux-gnu, adn with builsd for
the affected ABIs.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>