vl.c: In qemu -h output, only print options for the arch we are running as

Only print options in the help output that are accepted by our arch.
This is less confusing for users and also for other programs that
consume the help output.

The options affected are:

 -g and -prom-env only displayed on PPC or SPARC

 -win2k-hack, -rtc-td-hack, -no-fd-bootchk, -no-acpi, -no-hpet,
 -acpitable, -smbios only displayed on i386

 -semihosting only displayed on ARM, M68K or XTENSA

 -old-param only displayed on ARM

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
