ArmPkg/Semihostlib: Rename semi-hosting open option

ArmPkg/Semihostlib: Rename semi-hosting open option

Change mnemonic SEMIHOST_FILE_MODE_CREATE with mnemonic SEMIHOST_FILE_MODE_UPDATE.
The bit referred to by this mnemonic is for the semi-hosting open mode code the
equivalent of the + in the ISO C fopen mode terminology. This allows to select the
so called update mode for which both read and write are allowed on the open file.
The mnemonic SEMIHOST_FILE_MODE_UPDATE is more in line with the ISO C fopen mode
terminology. A description of the ISO C fopen modes can be found here :
http://pubs.opengroup.org/onlinepubs/009695399/functions/fopen.html.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ronald Cron <ronald.cron@arm.com>
Reviewed-By: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16237 6f19259b-4bc3-4df7-8a09-765794883524