CM3: trim the startup IRQ vector table.

CM3: trim the startup IRQ vector table.

The startup IRQ vector table will be replaced by another one in RAM
after the IRQ module initialization and the reset handler disables IRQ
at the very beginning, so we can safely trim the rest of this table to
reduce the memory footprint and save some space in flash.

For example, for the lm3s1968 example:

<before>
   text    data     bss     dec     hex filename
  23772    1036   18140   42948    a7c4 images/lm3s1968.elf
<after>
   text    data     bss     dec     hex filename
  23596    1036   18140   42772    a714 images/lm3s1968.elf

Moreover, make the vector IRQ table more generic (common for all the
Cortex-M3-based boards).

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@3410 38d2e660-2303-0410-9eaa-f027e97ec537