MIPS: Loongson32: Fix config brokenness; select SYS_SUPPORTS_32BIT_KERNEL

Commit a96d68ba ("MIPS: Loongson32: clarify we don't support MIPS16
and merge configs") attempted to reduce duplication in Kconfig by
consolidating some selects common to Loongson 1B & 1C CPUs under
CPU_LOONGSON1. Unfortunately it clearly wasn't tested because by
removing SYS_SUPPORTS_32BIT_KERNEL it prevented 32BIT from being enabled
leading to all sorts of strange build errors from a kernel configured to
build as neither 32 nor 64 bit.

Both loongson1b_defconfig & loongson1c_defconfig failed to build due to
this problem.

Revert the cleanup portions of commit a96d68ba

 ("MIPS: Loongson32:
clarify we don't support MIPS16 and merge configs"), keeping only its
removal of the selection of SYS_SUPPORTS_MIPS16.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Fixes: a96d68ba ("MIPS: Loongson32: clarify we don't support MIPS16 and merge configs")