bswap: Add new stn_*_p() and ldn_*_p() memory access functions

There's a common pattern in QEMU where a function needs to perform
a data load or store of an N byte integer in a particular endianness.
At the moment this is handled by doing a switch() on the size and
calling the appropriate ld*_p or st*_p function for each size.

Provide a new family of functions ldn_*_p() and stn_*_p() which
take the size as an argument and do the switch() themselves.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180611171007.4165-2-peter.maydell@linaro.org

docs/devel/loads-stores.rst

@@ -53,9 +53,24 @@ The ``_{endian}`` infix is omitted for target-endian accesses.
 The target endian accessors are only available to source
 files which are built per-target.
 
+There are also functions which take the size as an argument:
+
+load: ``ldn{endian}_p(ptr, sz)``
+
+which performs an unsigned load of ``sz`` bytes from ``ptr``
+as an ``{endian}`` order value and returns it in a uint64_t.
+
+store: ``stn{endian}_p(ptr, sz, val)``
+
+which stores ``val`` to ``ptr`` as an ``{endian}`` order value
+of size ``sz`` bytes.
+
+
 Regexes for git grep
  - ``\<ldf\?[us]\?[bwlq]\(_[hbl]e\)\?_p\>``
  - ``\<stf\?[bwlq]\(_[hbl]e\)\?_p\>``
+ - ``\<ldn_\([hbl]e\)?_p\>``
+ - ``\<stn_\([hbl]e\)?_p\>``
 
 ``cpu_{ld,st}_*``
 ~~~~~~~~~~~~~~~~~