rts: Consolidate RtsSymbols from libc

Previously (9ebda74e) environ was added to RtsSymbols to ensure that environment was correctly propagated when statically linking. However, this introduced #20577 (closed) since platforms are inconsistent in whether they provide a prototype for environ. I fixed this by providing a prototype but while doing so dropped symbol-table entry, presumably thinking that it was redundant due to the entry in the mingw-specific table.

Here I reintroduce the symbol table entry for environ and move libc symbols shared by Windows and Linux into a new macro, RTS_LIBC_SYMBOLS, avoiding this potential confusion