toolchain-funcs.eclass: Add tc-get-compiler-type()

toolchain-funcs.eclass: Add tc-get-compiler-type()

Add a tc-get-compiler-type() function that can be used to identify
the compiler being used, using the preprocessor defines. Alike
gcc-*version() routines, it uses CPP (which in turn uses CC).

The major usage would be applying compiler-specific quirks and limiting
gcc version checks to compilers that actually are gcc, since e.g. clang
reports gcc version 4.2 -- which would incorrectly cause numerous gcc
version checks in ebuilds to fail.