Fix target/103910: missing GTY on x86_mfence causing PCH usage to ICE

With -O3 -march=opteron, a mfence builtin is added after the loop
to say the nontemporal stores are no longer needed. This all good
without precompiled headers as the function decl that is referneced
by x86_mfence is referenced in another variable but with precompiled
headers, x86_mfence is all messed up and the decl was GC'ed away.
This fixes the problem by marking x86_mfence as GTY to save/restore
during precompiled headers just like most other variables in
the header file.

Committed as obvious after a bootstrap/test on x86_64-linux-gnu.

gcc/ChangeLog:

	PR target/103910
	* config/i386/i386.h (x86_mfence): Mark with GTY.