default integer value limits to 128 bits with __int128 extensions, fix for [GH #38]

[mohitmishra786]

Fixes #38.

With a 128-bit host integer representation, MAX_INTEGER_VALUE, MIN_INTEGER_VALUE and MAX_UNSIGNED_INTEGER_VALUE stayed at their 64-bit values, so the overflow checks in const_ints.c rejected valid __int128 constant arithmetic ((__int128)1 << 100 times 4 gave "expression must have a constant value"). The macOS CMake configurations use this representation by default.

• targ_def.h: when INT128_EXTENSIONS_ALLOWED, the default limits are those of __int128_t / __uint128_t.
• defines_linux.h and the Linux base.cmakedef no longer hard-code the 64-bit limits, so changing the integer value types to the 128-bit types also changes the limits. The limits are only used when INTEGER_VALUE_REPR_IS_A_HOST_INTEGER is TRUE, and the default Linux builds use the array representation, so their behavior is unchanged.

Testing (Docker, linux-gcc-debug):

• A build with INTEGER_VALUE_REPR_IS_A_HOST_INTEGER=1 and the __int128 types reproduced the error before the change. After it, valid __int128 arithmetic is accepted and real overflows (MAX + 1, MIN / -1, MAX * 2, MIN * 2) are still diagnosed, the same as the default build.
• changes/ and all tests using __int128: no differences from the expectations in edg_x86_64 and edg_x86_64_cp.
• linux-i686-gcc-debug builds.

[mohitmishra786]

@wchilders-nvidia @nv-cmeerw @daveedvdv could you please review this PR