[8u] RFR(XS): 8233880: Support compilers with multi-digit major version numbers

[Severin Gehwolf]

Hi,

Please review this OpenJDK 8u backport of JDK-8233880. It's a
one-liner change which updates the toolchain.m4 code so as to
recognize multi-digit GCC versions. For example Fedora 32 comes
with GCC 10 and falls afoul this check. As a result, a configure
warning is being produced and crucial flags won't get added for
compilation. This results in a broken build on such systems.

Details are in:
https://bugs.openjdk.java.net/browse/JDK-8244538

The original JDK 14 patch doesn't apply cleanly post path-
unshuffeling, because even though JDK-8151841[1] is in 8u,
JDK-8034788[2] later regressed the
COMPILER_VERSION_NUMBER sed expression to its prior
JDK-8151841 form. After this patch, this will be corrected again
and will include JDK-8233880 changes.

Bug: https://bugs.openjdk.java.net/browse/JDK-8233880
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8233880/01/webrev/

Testing: Build prior this patch on F32 with -fcommon (crashes on java
         -version). Passes after.

Thoughts?

Thanks,
Severin

[1] https://bugs.openjdk.java.net/browse/JDK-8151841
[2] https://bugs.openjdk.java.net/browse/JDK-8034788