gcc-common: Don't use "is" for comparing strings, use "=="

gcc-common: Don't use "is" for comparing strings, use "=="

Needed because the equality check was failing here even though upon
printing the LHS and RHS were the same.
As per http://stackoverflow.com/a/2987975/64537, using "is" compares the
memory addresses of the two objects which is not what we want here. We
just want to compare the values.

(From OE-Core rev: 7fd82cc90a48302ed42b6bfa962bb0de2c652b45)

Signed-off-by: Kartik Mohta <kartikmohta@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>