Move a private x86-64 header file out of the limelight.

Currently this is on everyone's include path and is getting exported
via the NDK.

Change-Id: Ia417839fb65c7e0afddcd9fae3438665dae8e752

diff --git a/libc/arch-x86_64/include/machine/fpu.h b/libm/include/amd64/machine/fpu.h
similarity index 100%
rename from libc/arch-x86_64/include/machine/fpu.h
rename to libm/include/amd64/machine/fpu.h