[dev.cmdgo] cmd: support space and quotes in CC and CXX

The CC and CXX environment variables now support spaces and quotes
(both double and single). This fixes two issues: first, if CC is a
single path that contains spaces (like 'c:\Program
Files\gcc\bin\gcc.exe'), that should now work if the space is quoted
or escaped (#41400). Second, if CC or CXX has multiple arguments (like
'gcc -O2'), they are now split correctly, and the arguments are passed
before other arguments when invoking the C compiler. Previously,
strings.Fields was used to split arguments, and the arguments were
placed later in the command line. (#43078).

Fixes golang/go#41400
Fixes golang/go#43078

Change-Id: I2d5d89ddb19c94adef65982a8137b01f037d5c11
Reviewed-on: https://go-review.googlesource.com/c/go/+/334732


Trust: Jay Conrod <jayconrod@google.com>
Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>