[libcc1] Improve detection of triplet on compiler names

[Sergio Durigan Junior <sergiodj at redhat dot com>]

Hi there,

This is a series of two patches, one for GDB and one for GCC, which aims
to improve the detection and handling of triplets present on compiler
names.  The motivation for this series was mostly the fact that GDB's
"compile" command is broken on Debian unstable, as can be seen here:

  <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=851146>

The reason for the failure is the fact that Debian compiles GCC using
the --program-{prefix,suffix} options from configure in order to name
the compiler using the full triplet (i.e., Debian's GCC is not merely
named "gcc", but e.g. "x86_64-linux-gnu-gcc-7"), which end up naming the
C_COMPILER_NAME and CP_COMPILER_NAME defines with the specified prefix
and suffix.  Therefore, the regexp being used to match the compiler name
is wrong because it doesn't take into account the fact that the defines
may already contain the triplets.

The GCC patch improves the libcc1::compiler_triplet_regexp::find and
libcp1::compiler_triplet_regexp::find methods by first trying to match
the triplet in the compiler name and correctly discarding the triplet
part of the regexp if the matching succeeds.  I've had to do a few
modifications on the way the regexp's are built, but I'll explain them
in the patch itself.

The GDB patch is very simple: it adds the trailing "-" in the triplet
regexp.  Therefore, we will have a regexp that truly matches the full
triplet (e.g., "^(x86_64|i.86)(-[^-]*)?-linux(-gnu)?-") instead of one
that leaves the trailing "-" match to libcc1.

I've tested this patch both on my Fedora and my Debian machines, and
both now work as expected, independently of the presence of the triplet
string in the compiler name.  I am sorry about the cross-post, but these
patches are really dependent on one another.

Thanks,

-- 
Sergio
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