[arm] Fix syscalls.c for newlib embedded syscalls builds

[arm] Fix syscalls.c for newlib embedded syscalls builds

Newlib has a build configuration where syscalls can be directly
embedded in the newlib library rather than relying on libgloss.

This configuration was broken recently by an update to the libgloss
support for Arm that was not propagated to the syscalls interface in
newlib itself.  This patch restores the build.  It's essentially a
copy of https://sourceware.org/ml/newlib/2018/msg00128.html but there
are some other minor cleanups and changes that I've made at the same
time.  None of those cleanups affect functionality.

The prototypes of the following functions have been updated: _link,
_sbrk, _getpid, _write, _swiwrite, _lseek, _swilseek, _read and
_swiread.

Signed-off-by: Richard Earnshaw <Richard.Earnshaw@arm.com>