[AArch64] Enable libm vectorized functions via SLEEF

This changeset is modeled after Intel's submission for SVML. It enables
trigonometry functions vectorization via SLEEF: http://sleef.org/.

 * A new vectorization library enum is added to TargetLibraryInfo.h: SLEEF.
 * A new option is added to TargetLibraryInfoImpl - ClVectorLibrary: SLEEF.
 * A comprehensive test case is included in this changeset.
 * In a separate changeset (for clang), a new vectorization library argument is
   added to -fveclib: -fveclib=SLEEF.

Trigonometry functions that are vectorized by sleef:

acos
asin
atan
atanh
cos
cosh
exp
exp2
exp10
lgamma
log10
log2
log
sin
sinh
sqrt
tan
tanh
tgamma

Patch by Stefan Teleman
Differential Revision: https://reviews.llvm.org/D53927

llvm-svn: 347510