Use gcc10 by default for Oracle Linux 7 and 8.

On el7, look for /opt/rh/devtoolset-10 (keep /opt/rh/devtoolset-9 for ARM)
On el8, look for /opt/rh/gcc-toolset-10

Downgrade -Wunused-function from error to warning when building a test
executable. (gcc10 bug?)

Building with gcc10 and PGO gives lots of warnings about
-Walloc-size-larger-than=PTRDIFF_MAX. Silence these when linking mysqld.

Fixed some -Wmaybe-uninitialized warnings when building with PGO.

Fixed a -Werror=stringop-truncation warning.

Downgrade -Werror=restrict to warning for an ndbcluster test executable.

Change-Id: I692195692f9edf4c61896fa94e0a2075ed5cdbcc
(cherry picked from commit 8229aae56302e291a18eff69f57cbca9c3b5df7e)