Remove the limit and solve https://gcc.gnu.org/bugzilla/show_bug.cgi?id=14940.

I have tested the patch on `x86_64-w64-mingw32` with MSVCRT, by pre-compiling a header of many 
standard and boost headers to generate a 313-MiB-large .gch file and using it to compile a simple 
'hello world' program, and seen no problems so far.


After eighteen years this has eventually been settled. (sigh)


-- 
Best regards,
LIU Hao