[Win32] define HAS_TRUNC and $Config{d_trunc)for MSVC builds

[sisyphus]

This PR might well be approved simply on the basis that supported MSVC compilers now provide the trunc function, but there's a little bit more to it.

Following the merging of PR #21610, ext/POSIX/t/math.t now fails 5 tests - though this happens only on 32-bit perl-5.39.5 (MSWin32-x86-multi-thread), built using MSVC.
Here are the failures:

....
ok 131 - nan + getpayload 2**31
not ok 132 - nan + getpayload 2**32
#   Failed test 'nan + getpayload 2**32'
#   at \comp-1320-ucrt\perl-5.39.5\ext\POSIX\t\math.t line 231.
#          got: '2147483648'
#     expected: '4294967296'
not ok 133 - nan + getpayload 2**33
#   Failed test 'nan + getpayload 2**33'
#   at \comp-1320-ucrt\perl-5.39.5\ext\POSIX\t\math.t line 232.
#          got: '2147483648'
#     expected: '8589934592'
ok 134 - nan + getpayload 2**31-1
not ok 135 - nan + getpayload 2**32-1
#   Failed test 'nan + getpayload 2**32-1'
#   at \comp-1320-ucrt\perl-5.39.5\ext\POSIX\t\math.t line 236.
#          got: '2147483648'
#     expected: '4294967295'
not ok 136 - nan + getpayload 3**21
#   Failed test 'nan + getpayload 3**21'
#   at \comp-1320-ucrt\perl-5.39.5\ext\POSIX\t\math.t line 258.
#          got: '2147483648'
#     expected: '10460353203'
not ok 137 - nan + getpayload prime
#   Failed test 'nan + getpayload prime'
#   at \comp-1320-ucrt\perl-5.39.5\ext\POSIX\t\math.t line 260.
#          got: '2147483648'
#     expected: '4294967311'
ok 138 - nan (trunc) + getpayload
....

Prior to the merging of #21610, these failing tests were skipped on this platform.
There's no problem with MSWin32-x86-multi-thread-64int - the failures occur only when ivsize is 4.
And there's no problem with the comparable mingw-w64 builds.

By defining HAS_TRUNC (in win32/config_H.vc) and d_trunc (in win32/config.vc), these tests all pass on the problem platform.
Importantly, with these changes in place, the MSVC builds of the MSWin32-x86-multi-thread-64int and MSWin32-x64-multi-thread configurations still continue to pass all tests.

I have tested this using configurations of current blead built with VS2022.