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 <drambo@broadcom.com>
Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Mark Hambleton <mark.hambleton@arm.com>
Cc: Tom Rini <trini@ti.com>
Acked-by: Steve Rae <srae@broadcom.com>
Suggested-by: Mark Hambleton <mark.hambleton@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>