manywarnings: port to GCC 8.0

manywarnings: port to GCC 8.0

* build-aux/gcc-warning.spec: Add -Wcatch-value,
-Wclass-memaccess, -Wdo-subscript, -Wextra-semi.  Adjust to the
fact that the GCC help message now mentions operands for
-Warray-bounds, -Wformat, -Wformat-overflow, -Wformat-truncation,
-Wimplicit-fallthrough, -Wplacement-new, -Wshift-overflow,
-Wstrict-aliasing, -Wstrict-overflow, -Wstringop-overflow,
and -Wunused-const-variable.
* m4/manywarnings.m4 (gl_MANYWARN_ALL_GCC): Add -Wattribute-alias,
-Wcast-align=strict, -Wcast-function-type, -Wif-not-aligned,
-Wmissing-attributes, -Wmultistatement-macros,
-Wpacked-not-aligned, -Wsizeof-pointer-div, -Wstringop-truncation,
-Wsuggest-attribute=cold, -Wsuggest-attribute=malloc.