target/arm: expose CPUID registers to userspace

target/arm: expose CPUID registers to userspace

A number of CPUID registers are exposed to userspace by modern Linux
kernels thanks to the "ARM64 CPU Feature Registers" ABI. For QEMU's
user-mode emulation we don't need to emulate the kernels trap but just
return the value the trap would have done. To avoid too much #ifdef
hackery we process ARMCPRegInfo with a new helper (modify_arm_cp_regs)
before defining the registers. The modify routine is driven by a
simple data structure which describes which bits are exported and
which are fixed.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20190205190224.2198-3-alex.bennee@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>