There was a duplicate definition of LDBL_MAX_EXP, which this commit
removes.
There was a missing definition for LDBL_MAX_10_EXP, which this
commit adds.
I've confirmed the value of LDBL_MAX_10_EXP using gcc on an AARM64
Debian system (gcc 8.3.0). I've only indirectly confirmed the value
on RISCV (using "#if ... #error" on godbolt.org). I'm reasonably
sure this update is correct, but someone should confirm it before
merging to "mob".
when x86-64 support was added, "for long double, we use x87 FPU".
And indeed, tests show that Intel's extended precision is used,
not double precision.
