[LINKER] Add single underscore to all definitions, no -D for dashed param.

This makes possibility for the script to append the definitions to CFLAGS
and LDFLAGS, and with the feature of disabling of -D prependation it will
make possible to generate ARCH_FLAGS generic to each specific chip.

ld/tests/dash.data

@@ -0,0 +1 @@
+dash END A B C D -mcpu=cortex-m0