tags, powerpc: Update tags.sh to support _GLOBAL symbols

On PowerPC we use _GLOBAL throughout the assembly to define symbols, but
currently these symbols are missing from the tags generated with
ARCH=powerpc make tags. This patch modifies the tags.sh script to
recognise _GLOBAL(.*) so that these symbols will be in the tags.

This is almost (but not quite) PowerPC specific and this change should
not affect anyone else:

$ git grep -E '^_GLOBAL\(([^)]*)\).*' |sed 's/^\([^/]*\/[^/]*\)\/.*$/\1/'|uniq -c
    627 arch/powerpc
      2 arch/um

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>