libc: Remove deprecated register

In c++11, register has been deprecated, and
libc is now built as gnu++11

From the documentation:
A register specifier is a hint to the implementation
that the variable so declared will be heavily used.
[ Note: The hint can be ignored and in most implementations
it will be ignored if the address of the variable is taken.
This use is deprecated (see D.2)

Change-Id: I459dc3f5f9de63fc09eeda3bc6700f31bdf20f6f