linking with lld (on mingw) fails due to a workaround for an old gcc bug

The workaround for a bug in GCC < 6.3 in the SSE version of PacketMath.h for pmax() is applied when compiling with clang 12 (on mingw64). This causes lld to fail during linking to due to the inline assembly at line 525.

Appending "&& !EIGEN_COMP_CLANG" at the end of line 516 allows the lld linker to finish.