software, integration/export: rename and reimplement CSR accessors

software, integration/export: rename and reimplement CSR accessors

Implement CSR accessors for all standard integer types and
combinations of subregister alignments (32 or 64 bit) and
sizes (i.e., csr_data_width 8, 16, or 32).

Rename accessors to better reflect the size of the register
being accessed, and correspondingly update the generation
of "csr.h" in "export.py".

Additionally, provide read/write accessors that superimpose arrays
of standard unsigned C types over a CSR register (which may itself
be spread across multiple subregisters).

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>