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: