If you get following compilation error while emerging xen (resp. xen-tools):

arch/i386/core/cpu.c: In function 'get_cpuinfo':
arch/i386/include/bits/cpu.h:79:2: error: can't find a register in class 'BREG' while reloading 'asm'
arch/i386/include/bits/cpu.h:79:2: error: can't find a register in class 'BREG' while reloading 'asm'
arch/i386/include/bits/cpu.h:79:2: error: can't find a register in class 'BREG' while reloading 'asm'
arch/i386/include/bits/cpu.h:79:2: error: can't find a register in class 'BREG' while reloading 'asm'
arch/i386/include/bits/cpu.h:79:2: error: 'asm' operand has impossible constraints
arch/i386/include/bits/cpu.h:79:2: error: 'asm' operand has impossible constraints
arch/i386/include/bits/cpu.h:79:2: error: 'asm' operand has impossible constraints
arch/i386/include/bits/cpu.h:79:2: error: 'asm' operand has impossible constraints
make[7]: *** [bin/cpu.o] Error 1
make[7]: *** Waiting for unfinished jobs....

Than you should not only switch to non-hardened gentoo profile as everywhere is written (via eselect profile), but you need also to switch to vanilla gcc compilator (via gcc-config). Doing just one thing is not enough – thats the magic. You can switch back your profile & gcc back when xen compiled.