Thumb2 SP ASM: mont_sub fix #4680

Job	Run time
build library (--enable-smallstack --disable-asm --enable-cryptonly --enable-all-crypto --disable...	43s
build library (--enable-cryptonly --enable-all-crypto --disable-examples --disable-benchmark --di...	45s
build library (--disable-asm --enable-cryptonly --enable-all-crypto --disable-examples --disable-...	54s
build library (--enable-smallstack --enable-intelasm --enable-cryptonly --enable-all-crypto --dis...	52s
build library (--enable-cryptonly --enable-all-crypto --disable-examples --disable-benchmark --di...	1m 3s
build library (--enable-intelasm --enable-cryptonly --enable-all-crypto --disable-examples --disa...	51s
	5m 8s

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