file: improve the file access interface

file: improve the file access interface

A number of fairly common operations are invoked way too many times,
especially when using incbin.  Drastically reduce the number of system
calls that need to be executed, and use memory mapping to reduce
unnecessary double buffering.

We could improve this further by leaving files open once used;
however, that might run into file count problems on some systems.

Still unclear is why we seem to invoke nasm_file_size() twice per pass
for incbin.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>