eal: fix compiler detection in public headers

eal: fix compiler detection in public headers

RTE_TOOLCHAIN_* macros are defining the target compiler.
Some API may be used in a host application like pmdinfogen.
That's why the the public headers should check the effective compiler
in use instead of the target compiler.

Detecting the compiler with macros is easy, except for __GNUC__
which is defined in all compilers supporting some GNU compatibility.

It is improved by defining RTE_CC_CLANG, RTE_CC_ICC and RTE_CC_GCC.
The extra macro RTE_CC_IS_GNU is defined to 0 or 1 in GCC case,
so it can be used simply with #if.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>