arm: semihosting: fix up compile bugs

arm: semihosting: fix up compile bugs

There is currently a regression when using newer ARM64 compilers
for semihosting: the way long types are inferred from context
is no longer the same.

The semihosting runtime uses long and size_t, so use this
explicitly in the semihosting code and interface, and voila:
the code now works again.

Tested with aarch64-linux-gnu-gcc: Linaro GCC 4.9-2014.09.

Cc: Darwin Rambo <[email protected]>
Cc: AKASHI Takahiro <[email protected]>
Cc: Mark Hambleton <[email protected]>
Cc: Tom Rini <[email protected]>
Acked-by: Steve Rae <[email protected]>
Suggested-by: Mark Hambleton <[email protected]>
Signed-off-by: Linus Walleij <[email protected]>