wireguard-tools: allow compiling with MIPS16 instructions

wireguard-tools: allow compiling with MIPS16 instructions

The wg utility compiles and runs without issues in MIPS16 mode, despite setting
PKG_USE_MIPS16:=0 in the makefile. Let's remove this, allowing for a substantial
size reduction of the wg executable. Since wg is a just a configuration utility,
it shouldn't be performance-critical, as the crypto heavy-lifting is done on the
kernel side.

wg sizes for both modes:

MIPS32: 64309 bytes
MIPS16: 42501 bytes

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>