emulators/simh-hp3000: fix the build with LLVM 11.

Similar to the patch for emulators/simh-hp2100, drop an -fcommon in to fix
the build in the face of GCC 10 / LLVM 11 switching the default to
-fno-common.

Minor cleanup while we're here, remove USES= compiler and just use ${CC} for
the compiler.

PR: 248872
MFH: 2020Q3 (-fno-common build fix)