Thumb2 SP ASM: mont_sub fix #3018

Job	Run time
make check (--disable-oldtls --disable-tls13 --enable-psk -disable-rsa --disable-dh -disable-ecc ...	38s
make check (--enable-psk C_EXTRA_FLAGS=-DWOLFSSL_STATIC_PSK --disable-rsa --disable-ecc --disable...	46s
make check (--disable-oldtls --disable-tlsv12 --enable-tls13 --enable-psk -disable-rsa --disable-...	50s
	2m 14s

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