Drop -msse4 compiler flag

Older CPUs that do not have SSE4.1 would crash with the Ruby native gem
due to an illegal instruction exception.

The Abseil random library isn't being used at the moment
(grpc#26476), and there's no reason gRPC
needs to force SSE4.1 instructions on all platforms at the moment. There
are other hardware-specific issues that need to be ironed out for this
to work: grpc#26479

When the `-msse4` compiler flag was enabled, the Abseil code started
using the `pinsrb` instruction:

```
$ elfx86exts abseil-cpp/absl/time/internal/cctz/src/time_zone_libc.o
MODE64 (ret)
CMOV (cmovne)
SSE2 (movdqa)
SSE41 (pinsrb)
SSE1 (movaps)
CPU Generation: Penryn
```

This was previously needed because gcc 4.8 wouldn't compile without the
`-msse4` and `-maes` flags.

However, per
gcc-mirror/gcc@97db2bf
gcc 5.0+ automatically detects whether these options are enabled.

clang still needs `-maes` since including `wmmintrin.h` expects the AES
option to be enabled.

Closes grpc#27095

templates/Makefile.template

@@ -442,7 +442,7 @@
   GRPC_ABSEIL_DEP = $(LIBDIR)/$(CONFIG)/libgrpc_abseil.a
   GRPC_ABSEIL_MERGE_LIBS = $(LIBDIR)/$(CONFIG)/libgrpc_abseil.a
   ifeq ($(HOST_IS_X86_64),true)
-  ABSL_RANDOM_HWAES_FLAGS = -maes -msse4
+  ABSL_RANDOM_HWAES_FLAGS = -maes
   else
   ABSL_RANDOM_HWAES_FLAGS =
   endif