Changing the default i387 precision

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

Hi all,
one of the missing parts of our libm is proper long double support for
x86. This is somewhat complicated by the fact that our long double is
castrated by the default rounding to 53bit (a.k.a. double) mantissa
size. The historic reason is that forcing double rounding in code that
needs the precision to be exact requires care and in theory, it might
change results of code that doesn't expect it. Given that Linux and
FreeBSD default to the whole mantissa, I doubt that the second argument
is valid. It doesn't normally apply to amd64 anyway, since most
compilers will default to the faster SSE units for double arithmetic
anyway. This leads to the important question of whether we want to have
real long double or not. Opinions?

Joerg