Add run-time CPU detection and support for ARM architecture

Run-time CPU detection (RTCD) is enabled by default if target platform support
it.
It can be disable at compile time with --disable-rtcd option.

Add RTCD support for ARM architecture.

Thanks to Timothy B. Terriberry for help and code review

Signed-off-by: Timothy B. Terriberry <tterribe@xiph.org>
diff --git a/celt_headers.mk b/celt_headers.mk
index 1febcde..49b1bef 100644
--- a/celt_headers.mk
+++ b/celt_headers.mk
@@ -2,6 +2,7 @@
 celt/arch.h \
 celt/bands.h \
 celt/celt.h \
+celt/cpu_support.h \
 include/opus_types.h \
 include/opus_defines.h \
 include/opus_custom.h \
@@ -29,6 +30,7 @@
 celt/vq.h \
 celt/static_modes_float.h \
 celt/static_modes_fixed.h \
+celt/arm/armcpu.h \
 celt/arm/fixed_armv4.h \
 celt/arm/fixed_armv5e.h \
 celt/arm/kiss_fft_armv4.h \