support multiple .discard.* sections to avoid section type conflicts

gcc 4.4.4 will complain if you use a .discard section for both text and
data ("causes a section type conflict").  Add support for ".discard.*"
sections, and use .discard.text for a dummy function in the x86
RESERVE_BRK() macro.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>