soong: move -Wimplicit-fallthrough from cflags to cxxflags

The compiler upgrade to r370808 can now check C code for implicit
fallthrough. This is triggering a massive number of warnings throughout
external/ and the rest of the platform.

Revisit enabling this for C another day.

Bug: 139945549
Test: m
Change-Id: I8dfaedab78c6230b46e8a45a9e65106dec363380
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>