math: Add long double constant definitions

These constants are GNU libc extensions that are likely to be adopted
by the next POSIX revision [1]. The definitions can be verified in
a PARI-GP shell session:

• M_El: exp (1)
• M_LOG2El: log (exp (1)) / log (2)
• M_LOG10El: log (exp (1)) / log (10)
• M_LN2l: log (2)
• M_LN10l: log (10)
• M_PIl: Pi
• M_PI_2l: Pi / 2
• M_PI_4l: Pi / 4
• M_1_PIl: 1 / Pi
• M_2_PIl: 2 / Pi
• M_2_SQRTPIl: 2 / sqrt (Pi)
• M_SQRT2l: sqrt (2)
• M_SQRT1_2l: 1 / sqrt (2)

[1] https://austingroupbugs.net/view.php?id=828

Put these behind BSD_VISIBLE || XSI_VISIBLE >= 800 to future-proof
these changes. They shouldn't be defined at lower levels of XSI, but don't
have other XSI 800 stuff in place yet.

Signed-off-by: Collin Funk <collin.funk1@gmail.com>
Reviewed by: imp, allanjude
Pull Request: https://github.com/freebsd/freebsd-src/pull/1121