Handle mscorwks_unixexports.src with better regexp operator for compatibility

Handle mscorwks_unixexports.src with better regexp operator for compatibility

ver3:
According to the gawk manual, The '\S' operator (as shorthand for '[^[:space:]]')
matches any character that is not whitespace. However, '[ \t]' is better than
'\S' for Mac OSX.
(source - https://www.gnu.org/software/gawk/manual/html_node/GNU-Regexp-Operators.html )

ver2:
With ver1, the build-break is caused by different awk versions (by @jkotas).
(Revert PR - https://github.com/dotnet/coreclr/pull/4005)
To resolve the limitation of the ver1, @janvorli proposed that the
\S expression is better than [:space:] for the most compatible solution.

There are a lot of different AWK software (e.g. awk, gawk, mawk, nawk, etc)
and versions among the popular Linux distributions. This patch is a simply
way to handle 'mscorwks_unixexports.src' file with \S operator for more
compatibility amon the different awk versions.

ver1:
It's must be modified by gawk implementation that check it exactly
as a plain character (e.g., space, alnum) within the bracket expression
at ./coreclr/generateversionscript.awk. The [[:space:]] is more appropriate than
the [:space:]]. (PR - https://github.com/dotnet/coreclr/pull/3943)

Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Prajwal A N <an.prajwal@samsung.com>