tests/tcg/arm: fix up test-arm-iwmmxt test

We need to rename the source file to a .S so we can do a single-line
assemble and link invocation. We also specify the additional CFLAGS
for the compile as it's a non-standard ARM binary.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
[rth: force fpu configuration]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>