[libc] Fix implicit conversion error on rv32 (llvm#138622)

This patch fixes the following error on rv32 (and possibly other 32-bit
archs):

FPBits.h:760:40: error: implicit conversion loses integer precision:
'int' to 'size_t' (aka 'unsigned int')
[-Werror,-Wimplicit-int-conversion]
  760 |       result.set_significand(number >> -ep);

Fixes llvm#138425.

Author: mikhailramalho

libc/src/__support/FPUtil/FPBits.h

@@ -757,7 +757,7 @@ struct FPRepImpl : public FPRepSem<fp_type, RetT> {
       result.set_significand(number);
       result.set_biased_exponent(static_cast<StorageType>(ep + 1));
     } else {
-      result.set_significand(number >> -ep);
+      result.set_significand(number >> static_cast<unsigned>(-ep));
     }
     return RetT(result.uintval());
   }