loongarch, um, xtensa: get rid of generated arch/$ARCH/include/asm/param.h

For loongarch and xtensa that gets them to do what x86 et.al. are
doing - have asm/param.h resolve to uapi variant, which is generated
by mandatory-y += param.h and contains exact same include.

On um it will resolve to x86 uapi variant instead, which also contains
the same include (um doesn't have uapi headers, but it does build the
host ones).

Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>