Fix building "generic" TRMM kernel with CMake

[albertziegenhagel]

The CMake "TARGET_CORE" variables stores the "generic" target name in all lowercase letters, but gets compared to an all uppercase string, which results in the wrong TRMM kernel being selected.

This MR converts the TARGET_CORE to all uppercase before comparing its value to make sure case mismatches are not an issue in the future anymore.

The selection of the wrong TRMM kernel can be seen when compiling OpenBLAS with CMake and the option -DTARGET=GENERIC.

In this case the tests sblat3, dblat3, cblat3 and zblat3 all fail with an error similar to the following:

 ******* FATAL ERROR - COMPUTED RESULT IS LESS THAN HALF ACCURATE *******
           EXPECTED RESULT   COMPUTED RESULT
       1      0.186813          0.373626
 ******* STRMM  FAILED ON CALL NUMBER:
    506: STRMM ('L','U','N','U',  1,  1, 1.0, A,  2, B,  2)        .

which is basically the same what could be observed in #2257.

[martin-frbg]

Oops - thanks.