fix arm asm meson source build

meson does mot support output with paths; add a meson.build file in the arm
directory. The output files were being incorrectly placed in the celt/ directory.

Program arm/arm2gnu.pl found: YES (/var/media/DATA/home-rudi/LibreELEC.kernel11/build.LibreELEC-H3.arm-12.0-devel/build/opus-v1.5.1/celt/arm/arm2gnu.pl)
Configuring celt_pitch_xcorr_arm-gnu.S with command

../celt/meson.build:51:25: ERROR: configure_file keyword argument "output" Output 'arm/armopts.s' must not contain a path segment.

before:
celt/celt_pitch_xcorr_arm-gnu.S

after:
celt/arm/celt_pitch_xcorr_arm-gnu.S
celt/arm/armopts.s
celt/arm/armopts-gnu.S

Signed-off-by: Rudi Heitbaum <rudi@heitbaum.com>
Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>