install.texi: Bump newlib version for nvptx + gcn

Before, newlib 3.2 was required for amdgcn and 3.1 for nvptx.
Now recommended is 4.3.0 which was just released on 2023-01-20.

While currently the old versions would work fine, upcoming GCC
changes depend on a newer newlib. Thus, the minimal version is
bumped instead of just recommending the new version.

For GCN, the bump is in preparation for permitting non-threadlocal
stack variables and vectorized math functions - both scheduled for
GCC 13 and added to newlib in 4.3.0.

For nvptx, this includes an emulated clock (commit 6bb96d13a),
a calloc fix (5fca4e0f1) and changes to permit libgfortran to be
compiled with I/O support instead of only in minimal mode.
(Patch approved for GCC 13 but pending on a nvtpx patch,
which for which review is pending.)

gcc/ChangeLog:

	* doc/install.texi (amdgcn, nvptx): Require newlib 4.3.0.