arm: smh: Export semihosting functions

This exports semihosting functions for use in other files. The header is
in include/ and not arm/include/asm because I anticipate that RISC-V may
want to add their own implementation at some point.

smh_len_fd has been renamed to smh_flen to more closely match the
semihosting spec.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>