libstdc++: add casts to from_chars in <charconv> [PR111883]

This fixes

.../charconv: In function 'std::from_chars_result std::from_chars(const char*, const char*, _Float16&, chars_format)':
.../charconv:687:17: warning: converting to '_Float16' from 'float' with greater conversion rank
  687 |       __value = __val;
      |                 ^~~~~
.../charconv: In function 'std::from_chars_result std::from_chars(const char*, const char*, __gnu_cxx::__bfloat16_t&, chars_format)':
.../charconv:763:17: warning: converting to '__gnu_cxx::__bfloat16_t' {aka '__bf16'} from 'float' with greater conversion rank
  763 |       __value = __val;
      |                 ^~~~~

which was breaking a test:

FAIL: g++.dg/warn/Wstringop-overflow-6.C  -std=gnu++26 (test for excess errors)

	PR testsuite/111883

libstdc++-v3/ChangeLog:

	* include/std/charconv (from_chars): Add explicit casts.