BinUtils: Restore toolchain prefix detection from compiler name 'c++'

Since commit b12aec6c (BinUtils: prefer bin utils matching the
compiler version extension, 2020-12-11, v3.20.0-rc1~171^2) we recognize
`clangg++` and `g++` but not `c++`.  Restore `[gc]` in place of `g`
in the regex, as we had before that change.

Fixes: #22069
56 jobs for !6011 with cxxprefix in 64 minutes and 3 seconds (queued for 2 seconds)
latest detached
Status Name Job ID Coverage
  Build
passed build:centos6-x86_64 #5709667
cmake build docker linux manual

00:06:27

passed build:debian10-iwyu #5709664
cmake build docker linux

00:01:51

passed build:fedora33-ninja #5709668
cmake build docker linux linux-3.17 manual

00:01:49

passed build:fedora33-sphinx #5709666
cmake build docker linux linux-3.17

00:04:02

passed build:fedora33-tidy #5709665
cmake build docker linux linux-3.17

00:03:22

manual build:macos-x86_64-makefiles #5709670
cmake macos shell xcode-12.4 nonconcurrent manual
manual build:macos-x86_64-ninja #5709669
cmake macos shell xcode-12.4 nonconcurrent manual
manual build:windows-vs2019-x64-ninja #5709671
cmake windows shell vs2019 msvc-19.28-16.9 nonconcurrent manual
 
  Test
passed test:fedora33-ninja #5710062
linux docker cmake x11 linux-3.17

00:04:31

created test:macos-x86_64-makefiles #5709674
xcode-12.4 nonconcurrent cmake macos shell
created test:macos-x86_64-ninja #5709673
cmake macos shell xcode-12.4 nonconcurrent
created test:windows-vs2019-x64-ninja #5709675
cmake windows shell vs2019 msvc-19.28-16.9 nonconcurrent
failed test:fedora33-ninja #5709672
cmake docker linux linux-3.17 x11

01:00:04

 
  Test Ext
passed test:cuda10.2-nvidia #5709678
docker linux cmake cuda-rt

00:43:23

passed test:debian10-ninja #5709676
cmake build docker linux linux-3.17

00:02:38

passed test:fedora33-makefiles #5709677
cmake build docker linux linux-3.17

00:02:49

passed test:fedora33-ninja-multi #5709679
cmake build docker linux linux-3.17

00:07:01

created test:macos-x86_64-xcode #5709680
cmake macos shell xcode-12.4 concurrent
created test:windows-vs2019-x64 #5709681
cmake windows concurrent shell vs2019 msvc-19.28-16.9
 
  External
passed cdash

passed ghostflow-check-master

passed ghostflow-check-release

passed ghostflow-stager

passed cdash

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passed ghostflow-check-master

passed ghostflow-stager

passed ghostflow-stager

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