efi_loader: Change ptr arithmetics tcg eventlog buffer

Although ptr arithmetics are allowed with extensions in gcc, they
are not allowed by the C spec. So switch to (void *)(uintptr_t) instead

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>