Implement sincos, sincosf, and sincosl.
The primary benefit of these functions is that argument
reduction is done once instead of twice in independent
calls to sin() and cos().

• lib/msun/Makefile: . Add s_sincos[fl].c to the build. . Add sincos.3 documentation. . Add appropriate MLINKS.

• lib/msun/Symbol.map: . Expose sincos[fl] symbols in dynamic libm.so.

• lib/msun/man/sincos.3: . Documentation for sincos[fl].

• lib/msun/src/k_sincos.h: . Kernel for sincos() function. This merges the individual kernels for sin() and cos(). The merger offered an opportunity to re-arrange the individual kernels for better performance.

• lib/msun/src/k_sincosf.h: . Kernel for sincosf() function. This merges the individual kernels for sinf() and cosf(). The merger offered an opportunity to re-arrange the individual kernels for better performance.

• lib/msun/src/k_sincosl.h: . Kernel for sincosl() function. This merges the individual kernels for sinl() and cosl(). The merger offered an opportunity to re-arrange the individual kernels for better performance.

• lib/msun/src/math.h: . Add prototytpes for sincos[fl]().

• lib/msun/src/math_private.h: . Add RETURNV macros. This is needed to reset fpsetprec on I386 hardware for a function with type void.

• lib/msun/src/s_sincos.c: . Implementation of sincos() where sin() and cos() were merged into one routine and possibly re-arranged for better performance.

• lib/msun/src/s_sincosf.c: . Implementation of sincosf() where sinf() and cosf() were merged into one routine and possibly re-arranged for better performance.

• lib/msun/src/s_sincosl.c: . Implementation of sincosl() where sinl() and cosl() were merged into one routine and possibly re-arranged for better performance.

PR: 215977, 218300
Submitted by: Steven G. Kargl <sgk@troutmask.apl.washington.edu>
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D10765