linux-user: Use glib malloc functions in load_symbols()

Switch to using the glib malloc functions in load_symbols();
this deals with a Coverity complaint about possible
integer overflow calculating the allocation size with
'nsyms * sizeof(*syms)'.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
