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fix(math): restore two fdlibm lines dropped from atan2 and acos (#4359)

atan2 kept z = pi/2 + pi_lo/2 for |y/x| > 2^60 but not m &= 1, so
x < 0 evaluated pi - (z - pi_lo) and returned pi/2 one ulp too high.
acos for x > 0.5 skipped clearing the low word of df, so df * df was
not exact and the correction term c lost accuracy. Both now match
fdlibm and the js backend.

Co-authored-by: Kaida-Amethyst <ziyue.liu.vm@gmail.com>

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