runtime: fix softfloat64 add/sub for normal operands cancelling to subnormal

[omarsy]

fpack64 returns a wrongly-scaled subnormal when fadd64/fsub64 produce a
heavily-cancelled mantissa: subtracting two near-equal, opposite-sign normal
operands yields a mant0 far below 1<<mantbits64 while exp0 is still a
normal-range exponent. The denormal branch resets to (mant0, exp0) and only
right-shifts to align to the subnormal exponent, but with exp0 above the
subnormal range that shift goes the wrong direction, so the result is the
un-normalized cancellation mantissa at the wrong scale rather than the correctly
rounded subnormal.

Re-normalize the mantissa (left-shift) before the alignment loop. This is a
no-op for already-normalized callers (fmul64/fdiv64/conversions), so they are
unaffected; only the cancellation case is corrected. fpack32 has the same
structure and is fixed for parity.

This only manifests on softfloat targets (e.g. GOMIPS=softfloat), which is why
it has gone unnoticed on hardware-float platforms. A randomized differential
check against hardware found the previous code wrong on >50% of normal pairs
that cancel into a subnormal; with this change those cases match hardware.

Fixes #79964

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