RFR: 8265768 [aarch64] Use glibc libm impl for dlog, dlog10, dexp iff 2.29 or greater on AArch64.

[gregcawthorne]

Glibc 2.29 onwards provides optimised versions of log,log10,exp.
These functions have an accuracy of 0.9ulp or better in glibc
2.29.

Therefore this patch adds code to parse, store and check
the runtime glibcs version in os_linux.cpp/hpp.
This is then used to select the glibcs implementation of
log, log10, exp at runtime for c1 and c2, iff we have
glibc 2.29 or greater.

This will ensure OpenJDK can benefit from future improvements
to glibc.

Glibc adheres to the ieee754 standard, unless stated otherwise
in its spec.

As there are no stated exceptions in the current glibc spec
for dlog, dlog10 and dexp, we can assume they currently follow
ieee754 (which testing confirms). As such, future version of
glibc are unlikely to lose this compliance with ieee754 in
future.

W.r.t performance this patch sees ~15-30% performance improvements for
log and log10, with ~50-80% performance improvements for exp for the
common input ranged (which output real numbers). However for the NaN
and inf output ranges we see a slow down of up to a factor of 2 for
some functions and architectures.

Due to this being the uncommon case we assert that this is a
worthwhile tradeoff.

-------------

Commit messages:
 - Use glibc libm impl for dlog,dlog10,dexp iff 2.29 or greater on AArch64.

Changes: https://git.openjdk.java.net/jdk/pull/3510/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3510&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265768
  Stats: 85 lines in 7 files changed: 79 ins; 5 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3510.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3510/head:pull/3510

PR: https://git.openjdk.java.net/jdk/pull/3510