hppa: Fix feholdexcpt and fesetenv (Bug 18110).

The constraints in the inline assembly in feholdexcept and fesetenv
are incorrect. The assembly modifies the buffer pointer, but doesn't
express that in the constraints. The simple fix is to remove the
modification of the buffer pointer which is no longer required by
the existing code, and adjust the one constraint that did express
the modification of bufptr.

The change fixes test-fenv when glibc is compiled with recent gcc.